No secondary is fine if you don't want to save your yeast and DH. Yes, you can DH in the keg, but that's messy. You can also DH in the primary, but even with bags, some will still get into your yeast. Guess that's not a big thing either if you wash it.
Edit: I should add that I don't use a secondary all that much, because about the only beers I DH are larger beers and I don't save the yeast from those anyway.