The Computer promo card, or Batcomputer in Batman Fluxx, for example, increases both your Play and your Draw by 1. 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.
You need to take Play All But 1 as a unit into consideration: you can’t just break out the Play All, and apply the effects of the Computer, and then apply the …But 1 part.
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 count. It’s pretty easy to see whether something says you “may” do it, or if it just happens.