I'm fairly new at IDPA so my times don't reflect any loss I get from having a mag-disconnect. I'm not too partial on firing without a mag in anyway, but that's just me. I see the merit of having it and not having it and I think the OP pointed these out nicely. The question for people that carry is one of practicality. If a BG grabs for your gun, you're going into retention drill, either keeping it in the holster or keeping it pointed away from you if it clears the holster. You will either retain the gun or not. With the mag-release, you have the option of dropping the mag and rendering it useless to the BG and yourself until you slam it back in or replace it with your spare (you do carry a spare and practice, right?). Without the mag-release, you can deny follow-up shots by dropping the mag, but it only takes one to kill.
Comparatively speaking, how often will you actually need the ability to fire that one shot without the mag in? I think it's better to have on on a carry piece, but IDPA should allow for those with them to be modified to the same function as those without (i.e. changing the sear-block or simple removal) if they are also produced that way.
"There are two sides to every schwartz." Everything has it's good and bad sides.