Do I need a BEC?

[lincpimp]

OK, so if I got this right, a BEC or UBEC does 2 things:
1) limits the power to the servos so I don't fry them with 6S

It is a voltage regulator of sorts. It converts input voltage to a specific voltage to drive the sevos and receiver, usually 6v.

2) cuts power if battery voltage falls below 3V/cell.

No ubec I know of does this.

So, if I DO have this right, the question is: do I need one of these things if I use a MMM? If so, which one? Castle?

The MMM has a lvc (low voltage cutoff) and a bbuilt in bec that can handle 6s lipo input. So it will work without any other electronics, such as a ubec.