I agree with others, no tighter than 4.5" like the diamond pro table with the deep shelves. And even that may be too tight for 14.1 or banks. "Pool" is a collection of games involving a ball and a stick. Unlike snooker, where everyone plays only one game, pool has 14.1, onepocket, banks, 9 & 10 ball, rotation, Honolulu, Irish pool, etc. It's important to have a table suitable for all of these games. Pockets that are too tight really detract from the game, and some more than others.
For pockets under 4.25", I feel like it really limits even great players and it can severely stunt the progress of really good players. For anyone less than that, really tight pockets can just ruin their game. They will never be able to understand the spectrum of positions routes available by cheating the pocket on a "normal" table. Just food for thought.