When you read the forum you get the impression that all americans play on 4 inch pockets or below, sometimes. 4 inch pockets with deep shelves would be a disaster for pool, especially 14.1 and banks. Nobody wants to see a nicely hit ball end a run in 14.1 because of ridiculous pockets.
For tournaments with brand new equipment and cloth 4 1/4 or 4 3/8 inch pockets are probably ok. Those tables mostly have perfectly set up pockets and new cloth, and well hit balls will slide in. In the real world of pool hall play, pockets sometimes have slightly worn pocket facings or are cut at the wrong angle. The tighter the pocket, the more critical this becomes. Also playing on worn cloth and maybe balls that are worn etc means that the margin of error becomes too small.
People have this idea that if you shrink the target you automatically become more accurate. This is only true to a certain extent and only for the players whose mechanics are allready very good. Some (or most) people will probably never be accurate enough to run racks on a 4 inch, deep shelf pocket table unless they actually move in to the pool hall for 12 hours a day.
What usually happens is that the smart players learn to shy away from certain shots that would be considered to be a natural part of the game on a normal table. So the game will change instead. No more long banks, no more english draw shots along the rail. Long combinations? Forget it.
Some people will object to this by stating that these shots will go in, if hit perfectly. The problem is that nobody hits shots perfectly every time and the first to realize this are the better players. If you watch snooker you will soon discover that certain shots are never attempted (unless the frame is secure). That does not mean that these shots are impossible, players make these shots in practice all the time. They just play the percentages, that is all. So what will you achieve by tightening the pockets and deepening shelves? You will turn pool into a boring, tactical buntfest. Once the banks and power shots are taken away, safeties will become the bread and butter of the players. Congratulations, you just made pool even more boring and unwatchable for the general public.
Another thing is: Who are the ones who complain the most about the pockets being too lose? The player in the chair, that's who! I can't count the number of times I've made a run in straightpool only to hear the player in the chair complaining about the pockets being buckets etc. Well, we are playing on the same table, so why don't you run out the set, then?! Tight pockets are a solution to a problem that never existed. Nobody is running complete sets, even in 9 ball, if the races are to 9. And even if they did occationally do it, so what? Don't you want to see that, once in a rare while?