I agree, in theory the concept is interesting but in practice, it's well... impractical in my opinion. Simple SSIS lookup operations etc. work really well with dimensions, especially small ones.

I have read the same kind of arguments here a while ago: http://forum.kimballgroup.com/dimensional-modeling-and-data-architecture-f6/eliminate-date-dimension-surrogate-key-t334.htm

While it was an interesting read, I think these dimensions are very valuable nonetheless. I can easily ask what activity happened between hours etc. for all mondays etc. and use indexes while doing it. I know in Oracle for example you can index on a function but still...

Anyways, I think there are arguments for and against. But I think that providing both is a good thing. I can't really imagine somebody would put a date dimension and time dimension key in a fact without the datetime itself so having all of them kinda covers most scenarios I can imagine.