Personally, I think they should keep the numbers and the volumes and this is my reasoning.
What Millar and Hitch have given us here is more than just an incredibly witty political/social satire comic with great characterization, interactions, storylines and actions sequences, packaged with some of the best comic art ever produced. They've given us the future of how to produce ongoing series.
Ultimates is a fantastic read, and so far Ultimates 2 has been just as fun. While they are definitely interconnected and play off each other, it's possible to read both volumes as independent stories and enjoy them quite a bit. Read them altogether and it gets even better.
What I'm driving at is that both stories read like their own miniseries yet they function as an ongoing. THEY NEED TO RECOGNIZE AND KEEP THAT IDEA. It is the solution to the whole reason of why the Ultimate Universe exists. How do you do an ongoing series with the potential to last for years that will both please longtime fans but allow new readers to jump in and not be lost? You do an ongoing where each arc or storyline functions basically as a self-contained miniseries.
The Ultimates is the perfect book to do this with as well since being it is a team book with no (true) central character, you can pick and choose the focus, rearrange the roster, change the characters and do what you want to keep it interesting and tell the best story possible. That's my reasoning for why we should keep the numbers along with the volumes. Let each creative team do its stint, write one or two or three arc's, then bring in someone else and keep the ball rolling. That's my opinion at least.