Splits don't create holes, leverage points, play calling and talent do that. Splits DO make an impact and its something you can play with. For example, if the defense are playing landmark football (i.e., the Quick end plays a 7tech position and the NT plays a 1 Tech position) the boundary tackle should widen his splits and it'll push the Quick out even further and the tackle only has to wall off, while the guard can have a free release to the Will linebacker (in an zone scheme) and if you have a pulling backside blocker (TE, OG, etc) you'll have a fee blocker going up the field.
Typically, splits are 3 feet, I'm a fan or 2 foot splits between OC and OGs with 3.5 split between OTs and OG.
The benefit of tighter splits is, when it comes to pass pro is just turn into a big jumble of people if there are blitzes or stunt/twists. while int he run game, similar thing can happen and a small crease can turn into 12 yard burst or edges crash down inside more often and you hope the ball carrier can break contain and off to the races. The Cons are the same, a big can of smashes buttholes with nothing to work with and playing in a phone booth.
I'm a fan of space, that's why you're seeing a movement towards smaller tailbacks and those with more wiggle.
I agree they are tight, but its not the SOLE problem.