I think I remember this. We're probably referencing the same interview. If this is the one where suddenly Ed Stern has to go, and they continue interviewing Rahdo alone, I think Rahdo starts talking about how he hopes that they support changing to another of your persistent characters during a match, while waiting to respawn.
I don't think it's important for game mechanics to "make sense" as much as it is for them to be balanced. How much sense does it make that a character can absorb an unlimited amount of gunfire over time, as long as a medic supplies him with enough bandages and adrenaline? This is a game where Soldiers crap ammo and have some kind of magnetic field that draws gunfire away from their allies, Engineers are apparently so smart they can make your gun work better just by touching it, and Operatives can supposedly give themselves tattoos, change their skin color, and ... here we get back to Ed Stern's "doesn't make sense" moment in the interview ... change size. I'm not complaining about the unrealism; I actually like how SD thinks with all these mechanics. Realism is for simulations; when it has a negative impact on gameplay, it needs to be ignored. Considering the impact body type has on how you play the game, I am worried about the consequences of not allowing players to change it during a match.
As far as body types are concerned, it's not so much about "making sense" as it as about balance like you said - If I could change my body type whenever I want, what's the point of even having Body types? I look at is as one of those choices you make and have to deal with, like how in other games, certain weapons are restricted to certain classes - you want to be the Soldier, but you're restricted to using AR's - it's a choice you deal with. in Brink, I see Body types being the "restriction." You can still play any class regardless of your body, and there are no "body specific" objectives, so the ability to change body type at will really doesn't add more balance to the game. It adds a sort of depth and tactical choice if you can't change Body types at will.
The thing they were talking about where I could see it maybe not being so bad, is that you change characters, not just body types, so if you have a kick ass Heavy Medic in the game, and you want to go Light, but you have no Light's spec'd as a Medic, that's a compromise you would have to make.