UBEC is a switching voltage regulator, it takes the 11.1v the lipo puts out and makes it the 5~6v the radio/servo's need.
The ESC usually has a voltage regulator in it but it workis differently, it's a linear (not switching) regulator, so it has to bleed off some heat (wattage) to bring the voltage down to what it needs. A switching regulator turns on and off at a controlled rate to produce the proper voltage without much heat at all.
The bec in the ESC will get hotter the more volts it has to bleed off (heat you don't want in your ESC) using s UBEC like this
http://www.castlecreations.com/products/cc_bec.html will remove heat from the ESC.
Have no fear about getting the MM, lots of guys have run them in some heavy hitting MT's with good success, you'd be one of the few actually using it within specs hahaha. No worries.