I have to enthusiastically agree with this. The way it is now, there's no tactical use for cortex anymore. I can't count the number of times I've waited outside a door (with a baddie just on the other side) until boom-time, opened the door, and heard "deactivated" only to get killed by a single MP burst.
As for the deactivation itself, what's the point? How about a little boom that just kills the cortexer and delivers no damage to anyone around him? If I'm in combat and have my cortex activated, running around with 5 hp and no armor is just a waste of time for me. I'd rather just die and respawn.
It seems like a fair compromise would be toggling the implant key after activation to keep it from deactivating. That would allow the cortex to actually be used reliably, while keeping newbies from blowing up their whole team.