The question of escapism requires context in order to discern whether it is useful or detrimental. By itself it is unclear which way it should be considered as it has both a positive and negative aspect. For example, escapism when in jail, certainly, is a positive. However, escapism when reality requires your attention is a negative. As an example, those who escape into fantasy worlds (to the extreme) and wind up missing work, and (in some cases) living degraded lives in their mom's basement playing World of Warcraft... that would be rightfully considered a negative. So... whether escapism is useful and a good thing, or detrimental and a bad thing depends on who is using it, when, and for what purpose, and to what effect. In other words - it's complicated.
Stories are indeed important. However, they are not important because of their entertainment value, although that is what makes them useful (which is to say that is why people listen to them). They are important because they teach valuable lessons about life. For example, that prehistoric caveman around the fire telling the story of the wild buffalo and how he overcame it with his "new fangled" arrow (the older geezers probably grumbling about how in the old days they had to use their teeth to slay the buffalo) taught the youth that arrows are a good and heroic thing.
But furthermore, and this is where it gets really weird, the stories of the ancients very often dealt with completely impractical matters such as The Gods, and the dawn of existence, and this sort of thing. But laden in those stories were a lot of lessons about morality. And that's really why they were told, I think. Now we might look at those stories of The Gods and wonder what the heck kind of morality they had, seeing has how they were pretty wild, incestuous, and power-crazed (apparently), but we can't really fathom from here what the minds of our ancestors thought, and what their hearts felt, when they originally heard these stories for the first time. We can't possibly expect to superimpose our 21st century emotions, moralities, and thoughts on the distant past that way and imagine that we can understand any of their feelings or thoughts. We'd be bone headed stupid to assume we can understand any of it from their point of view.
Speaking of which... Imagination. Fantasy is all about imagination, and that's been going on since the dawn of man. But the reaction against it, which appears to culminate in the accusation of escapism, is unfairly targeted at the Industrial Age. In fact, warnings against the imagination are founded in none other than the Bible.
"And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually."Now some people might say "oh well that's why Religion is so bad... how can people cast aspersions on Imagination?!" But I think that would be an overly simplistic conclusion to draw. The ancients who wrote all of those stories that we so love include those that are found in the Bible, many of which actually predate the Bible and come from Summarian culture anyway. They are some of the oldest and most profound stories in the world. So we shouldn't lightly discount them as if we are all so smart. We should, I think, try to understand them as best we can. There may be hidden nuggets of wisdom there, after all.
- Genesis 6:5
But lets put Imagination in the Biblical context for a moment to see why, and how important it was. It was the imagination of man that produced, after all, the technological wonder of the Tower of Babel, which "reached the heavens". What this story is about, the lesson, I think, is that man is too immature, to petty, selfish, and cruel to obtain God-like Technological Power. It was, from God, a big warning when He cast down the Tower of Babel and thwarted the once unified language of man. It was to prohibit, perhaps, the ultimate evil - that the imagination of man would create technologies that could destroy the entire world. Or worse - the Universe itself (tampering with the boson higgs field comes to mind: http://www.livescience.com/47737-stephen-hawking-higgs-boson-universe-doomsday.html).
So the power of the Imagination is great... and it could be our ultimate doom. And that might be why the Bible warns us about it, and against its use.
But that takes me back to the Bible itself - a collection of stories. Which employ the use of the imagination. We visualize these stories in order to understand them, and God, apparently, gave us the ability and power to do so, as well as the Bible itself with which we are obviously intended to use our imaginations. So imagination, although unwieldy and potentially dangerous, is also intrinsic to our being. It is the God-like power that divinity grants to mankind, and that's how Tolkien saw it. Which is why he was an advocate of Mythopoeia. Again, just like with escapism, Imagination is complicated.
What Tolkien and CS Lewis, Owen Barfield, and the rest of the Inklings were concerned with was the use of the Imagination to produce Good, rather than evil. And as stories can, and should, teach valuable lessons, and not the least of which are lessons about morality, they should be considered to be a Good use of the Imagination. Just as we would have to conclude that the Bible itself is a Good use of the Imagination.
Another thing to consider that Jens D's post brought to mind is that Tolkien lived through both WWI and WWII... he fought in WWI on the front lines and at the Somme. He was a man who lived through the absolute worst that human imagination had been able to contrive to that date. We in our time, those of us who never lived through such things, have an almost impossible time understanding the world-view that was inevitably molded by such experiences. So when Tolkien rails against jailers and accuses them of being the ones who are most likely to denounce escapism, I think we should understand the context of his experience that might well have lead him to feel that in the big picture we are all in prison... with small and fruitless interludes between The Great Wars that actually define and are the reality of human life on earth. And we should also remember that he was a devout Christian, and therefore believed that the world itself is ruled by Satan, and that the true escape is to Heaven itself.
Which leads me to the last point I want to make - escapism in the service of the Good is a wonderful thing. But I can't say that it is always and absolutely a good thing. If escapism leads one to a state of depravity, which it very well can, then it's not a good thing at all. After all, one can easily jump out of the frying pan and into the fire.
So in conclusion, it is my assessment that when creating a world and adventures therein, that we also may keep these ideas in mind as well. It is my hope and goal to make my World and it's campaigns not merely entertaining, but at some level, albeit often imperceivably so, lessons of one sort or another. Whether I am successful at that is besides the point, as I can't really be the judge of the result. I'm far too biased in my own favor. So I will have to leave it for history to decide.