That's true, an exhalation valve can greatly increase user comfort. And if it helps keep moisture build up down inside the mask, it should help lengthen its usable life.

The only situation I can think of to avoid the exhalation valve is perhaps in a pandemic scenario where you may have some concerns that you may be the one potentially infected and you want to protect those around you, say you have caregiving duties over kids or elders.

Say you've gotten over something that seems like the pandemic disease and you need to get back on your feet but you're not sure if you're still shedding the virus from your lungs. Something like an N-95 will trap the virus particles better than a simple surgical mask. Certainly not foolproof, but it should help if you wear it properly.

That reminds me, I'd like to go see the new movie Contagion. Shhhh, if you've seen it, don't spoil the storyline for me!