Heck, if you are allowing multipowers to be changed multiple times in a phase then what is the point of not purchasing any powers that are not in a multipower. By having it that way, you could move, attack, and then bring up your total defenses, all in the same phase and all from the same multipower. So instead of paying 60 points for that 60 hex flight, plus 60 points for that 12d6 Blast, plus 60 points for that 20 PD 20 ED Resistant Protection, for a total of 180 points you only have to pay 78 points for the exact same effect with the following multipower:
Cost Powers
60 Multipower, 60-point reserve
6f 1) Resistant Protection (20 PD/20 ED) (60 Active Points)
6f 2) Blast 12d6 (60 Active Points)
6f 3) Flight 60m (60 Active Points)
That's one hell of a savings. Definitely no point in purchasing the powers individually.