Personally, I think speed is the main factor here. Although mecanum is maneuverable, I have a feeling that it is not ideal.
The faster you get across the field, the more pieces you can hang. Period. I think everyone can agree with that.
I have a prediction that all but maybe one of the robots on Einstein will have a 6wd, or 8wd, dropped center. Look at 2007....
http://www.thebluealliance.net/tbatv/event/2007cmp