In order for people to actually overspend with MM+ you have to have a working product ... which it currently is not. Based on our observations, speaking with people throught the parks and coupled with CM's input the system is a long way away from working flawlessly, in fact, most CM's we spoke with including those at Guest Relations didn't like them.
A lot of people we saw had abandoned the band and were pulling out their own plastic, room cards or simply good old fashioned cash. Most people we talked to were concerned about overspending with the 'ease of the band' and as a result were being more concious of what they were actually buying.
I know in our case this was the truth ... We typically spend on a 8 day trip anywhere from $1000 to $1500 on souviners, adult beverages and signature dining, separate from our Dining Plan or AP ... this trip our total was well under $1000, much to my surprise.