I don't see how exhaling in the last 5m or so is detrimental in any way. You still have to exhale at some stage before you can inhale so if that's done underwater it means you are actually getting your inhale in quicker. I wouldn't see any benefit doing it apart from the last few seconds before hitting the surface. Also PFI teaches it as a "relaxed" exhale so you probably hit the surface with about a half lungful.
I don't think anyone buys into the reverse O2 exchange ie the Terry Maas book "suction" theory anymore. Like Connor already pointed out exhaling before reaching the surface may reduce rapid blood pressure changes. This can have a big effect on blackouts if you are already hypoxic. It's the same sort of reason you don't want to jump up and stand after a static.
Obviously if you use packing then you want to exhale before reaching the surface to avoid lung injuries.
Cheers,
Wal
