I'm going to go a bit against the grain here.  While I agree that most transitions take away from the presentation, I consider them good in two ways:

  1. Beginning the presentation.  A good set of transitions and animations can grab the audience's attention.
  2. Quick fade between slides.  I prefer a quick fade as opposed to a snap; this is only distracting if it's too slow.  (0.25 seconds is good.)

EDIT:  I like them in one more way:  transition between major sections (of which there should be very few).  This gives the audience a better visual cue that the presentation has progressed to the next section.