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:
- Beginning the presentation. A good set of transitions and animations can grab the audience's attention.
- 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.