There are some 2-way games that use more than two though.
The one I can think of right off the top of my head is Bank Panic.
Then there are the 5-button games, that will play well enough, if you use the stick for left/right.
That leaves you needing three for thrust/fire/hyperspace on Asteroids.
(Yes, you CAN map those to the stick, but it sucks trying to PLAY them like that--that's the reason I got into this hobby in the first place, 2600 Asteroids felt wrong using a stick).