Typical multicade setups are one player. So if you're looking for ideas on a good single player layout, I think this layout works well.

Add another row of three buttons above that one and you've covered all of the Neo-Geo games, 6-button fighters, and a slew of other really great games too.
Personally, I totally understand the desire for that kind of project, mostly because
very few of my friends share in the enjoyment of the classics... (I know, I know... "get new friends!")

Sheer floor space for a full-size project can be an issue, like it is here in NYC...
Girlfriend/spouse/partner/whatever approval definitely comes up a lot, too...
Ergonomics regarding your available parts selection could also be a factor. I'd hate to see a full 2-player panel on a project relegated to a 19" (or less) monitor. That's CrapMAME waiting to happen...
I generally plan/build my projects with simultaneous multiplayer in mind, but yeah, if you're dealing with any of the above (or like boardjunkie said, you just want it that way!) there's something to be said for building 1-player and having a USB gamepad or some such stashed away on the occasion someone wants to join in.