I used
DCL air as an add-on with the cruise thru my TA. It's our first cruise and I wanted to play it safe, plus, I hadn't discovered these boards yet.
My flight from Denver is requiring a visit to Atlanta on the way, and Dallas on the way back. They used Delta, which does not have a nonstop from Denver. Had I booked my own on Frontier, it would have been nonstop and at the times of my choosing.
To figure out the answer to your question, if your home has a fairly easy route with Delta, then maybe they won't bounce you around any more than your own arraignments would. They 'might' use other airlines as well. Are they flexible? Sure, but it will cost you extra!
The only perk to using DCL air, that I have found, is the guarantee that they'll get you onto that ship one way or another. We'll fly in the day before, and they pay for the room at the MCO Hyatt, and the bus is covered, so that helps out with the cost comparison a bit too.
My biggest problem with the whole air situation was actually with my TA. They wouldn't call DCL to get me the flight numbers (so that I could get seat assignments) and said I needed to get it from DCL myself, back and forth etc... Finally a VERY NICE DCL rep heard my plea and made a conference call to my TA with me, and explained their job to them. SO, if you are booking air directly with DCL, than that won't be a hassle. You'll be able to skip the middle man and get info and docs directly from DCL, as well as accessing your reservation thru their website.
Maybe you could call DCL air (1-800-951-3532) and ask what airlines they would most likely use from your location. They won't give you any firm answer I'm sure, but if you only ask for the 'more likely' airlines, than you can see if any of them have the better routes or not. If none do, than there's your answer.
Next time I'll use D.U!
Good luck and have a great time!