I think you'll find that a lot of teams use the kit base. I've even heard of teams that certainly have the engineering ability to build their own custom base, just go with the kit base because of the reasons you have listed. Why reinvent the wheel(base)?
We got the AM14U3 and bought an AM14U2 kit at half price during one of AndyMark's sales. These were up and rolling within days of kickoff for prototyping and for software teams to start driving. We quickly realized that we wanted to upgrade the wheels to 8" pneumatics. Other than that and a change of frame configuration, it was pretty much as-is.
We do want to prototype other types of drive systems, but unless we hit a game that really needs it, I expect we'll keep using the kit and modifying it to suit.