How do you pay for powerups? Well, by sacrificing MWG lives, of course.

Some obvious tactics that arise from this:
-When a group of pretty useless MWGs is outnumbered and about to be overrun, you can simply c them before the enemy kills them and get powerups instead.
-Reaching the population limit, you can just c some guys to get powerups and the guys will be replaced at your spawns in no time.
-Assault attacking will become very easy indeed so probably not a good idea to enable this in Assault...
To make this a little less powerful and a little more possible to counter, what you buy could not be an instant powerup but instead a crate with known content which needs to be deployed at a choosen point by you, then fall and be researched. Players would then have to choose if they want to buy defenses against common attacks early to have them available the moment the attack comes. Otherwise it'd be a minute before that ant killer powerup is actually usable and you'd lose the MWGs for paying at the same time you lose a spawn to ants... Better to be able to react fast and spend MWGs when you can afford it but of course you only have so many task manager slots.