Have to admit that I haven't tested PowerPoint Mobile yet so can't comment on that, or indeed it's Docs To Go counterpart.
The included versions of Word and Excel may well be fine for most people. I just like having the extra functions of the Docs To Go apps should I need them.
Then again it's the same for most, if not all, of the built in applications. There are always similar programs available to buy with more functionality for those that want it. One of the reasons I like owning a Smartphone, even if it ends up costing me more money!
Jason.