I like to have the full information of what I'm doing all available without having to do the math in my head...
To me, that means I want to know how many photos will be taken at what interval, at what start and what end time.
eg, if I set "photos every 2 minutes, starting at 6am, stop after 500 photos" ...quick - what time will it finish? (thinking: ok, 1000 minutes is how many hours... uhm...? (this example is 16 hours 40 minutes)
...additional information makes it easier to have all the facts about what is about to happen, meaning one can spot possible errors in the setup quickly and easily :)
(I'd almost argue that one should also be able to set 'start time', 'stop time', and 'number of photos', and let TimeLapse work out the appropriate interval automatically, but I can't think of much general use for that (specific examples I can think of, but I don't think they make good arguments, so consider this whole paragraph as nothing more than thinking out loud :)