When Would You Prefer DateTime Over DateTimeOffset

Well, one obvious answer would be when you need to support clients without the SP that it ships in (it isn't actually in 3.5 - it is in 2.0 SP1, which shipped at the same time).

Sometimes you really just want to represent a "local" (timezone unaware) date and time rather than an instant in time. To be honest it's more often useful to represent just a time - e.g. "wake me up at 8am, regardless of timezone" - but date and time could be useful too.

I agree that for the vast majority of cases, DateTimeOffset is a better fit. It does strike me as odd that there isn't a DateTimeTimeZone struct which has both the instant and its timezone though... an offset doesn't actually give you all the information you need. (For instance, given a DateTimeOffset, you don't know what the time will be 24 hours later, because you don't know when DST might kick in.)

If you want that kind of structure, I have a very crude implementation in another answer. I'm sure it could be improved very easily :)