It definitely covers all consoles if you use P1 start as Start and a coin button as 'select'.  For what I wanted out of my cab, I wouldn't change anything.  I'm one of the few who like the larger sized cabs, and grew up playing 4 player games with strangers like the simpsons or x-men.
I laid out all my controls, and it didn't look overly crowded (which can be argued), so I figured why not?
However, I rarely use all 8 buttons and I play MAME games more than any other, but I do play other games occasionally and like having the functionality to be able to play those games.  Also, I have never had player 2 use all 8 buttons yet.  A multi-player game that uses all 8 buttons requires serious commitment to learn the game from the second party (which my arcade is generally used for shorter play sessions with guests).  But I needed the 8 buttons for player 2 for symmetry.
And I could of went down to 4 buttons for P3 and P4 instead of 6.  I was thinking it would be fun to play something like 4 player madden or something, so I went with the 6 buttons for P3 and P4, but they haven't been used yet.  But then again, I do have the functionality.