What Cheffo said. Keep these points in mind:
1) MAME doesn't ever need mouse buttons to play the games or start (unless it's MAME32 - but that's not cab friendly, see below.)
2) Putting mouse buttons for Windows applications (i.e. setup/maintenance) is unnecessary. That falls under "admin buttons", which really do not belong on a CP.
3) If another PC game or emulator requires mouse buttons, it's a fairly good chance it's not a candidate for an arcade cabinet anyhow.
4) If you really do need that mouse button, you can use programs like Joy2Key to register a left/right click when necessary (even using unseen "shifted" buttons some encoders have.)
5) Keeping a cheap mouse/keyboard inside the cab, or tucked away somewhere, or having a wireless set up is loads easier than drilling, buying and wiring extra buttons.
6) Etc. Etc.
Basically, if you had to ask, you're better off without them. They take up space unnecessarily when the main buttons work just fine.
As an aside, once you get used to it, you can pretty much maneuver around Windows without a mouse anyways for those quick little fixes.