I do BIAB in a cooler, and sometimes when doing a Big beer with a lot of grain, I reduce my water/grain ratio so it will fit the space I have.
Does it make a difference? Maybe, but maybe not. The bottom line is I'm happy with the way the beer comes out with the equipment I have so I'm not going to worry about it.
I do think more experimentation on this subject would be worthwhile and interesting. Different styles and grain bills could make for some different results.
As long as water can permeate the grain to release starches and enzymes, those enzymes will actually have an easier time finding the unconverted starches in a thicker mash (in reality it's virtually the same at all reasonable mash thicknesses).
The most convincing argument against thin mashes that I've heard is that you lose mash buffering ability and have to be more conscious of your pH balance. But that is mainly an issue when sparging anyway.