The Computer promo card, or Batcomputer in Batman Fluxx, for example, increases both your Play and your Draw by 1. So does the Rich Bonus. Play 1 extra doesn’t affect Play All, so why would it affect Play All But 1?

A: Play All But 1 isn’t the same as Play All: when you Play All But 1, you do have 1 card remaining, so when you are then required to play +1 cards, you do have one left to play, so you must play it.

In this case, we don’t separate the “Play All,” and apply the effects of the Computer for no effect, and then apply the “But 1” part. Note that Play All Plus 1 is functionally a bit different, so we DO break out the Play All from the Plus 1, since that extra card beyond what you have in your hand is

Note that since the effects of the Computer are not optional, you have to do it, whether you like it or not. Other similar effects may be optional, so always check on that. It’s pretty easy to see whether something says you “may” do it, or if it just happens.