I started having trouble around my 5th hatch. We got a new incubator for lockdown after killing the one we were using. I went from having 85 - 95% hatch rates to ZERO, to 10% hatch rates? So, I was looking to change EVERYTHING. We were going to just add the oxygen, but it was suggested we add warm moist air. It doesn't seem to impact the humidity much (if at all).
Unfortunately I added the air pump at the same time I made a lot of other changes, so I can't confirm the air pump helped. I just hatched the last 2 sets of eggs I had brewing. About day 10 on the first to hatch is when I had upped the temps in my incubator (day 6 for the next). That first batch, I had 5 out of 15 hatch, one had it's insides hanging out and died, and one had to be culled. The second set 4 out of 4 hatched healthy. So, I know my temps were an issue. But I do think the extra oxygen helped.
I was wondering if it wold hurt to keep it in throughout the entire process. I never add water the first 18 days - my house runs around 35%.
OK, so it's not for controlling humidity, it's to increase oxygen.
here is a link to the post about ventilation/oxygen. : http://www.backyardchickens.com/t/1046313/incubating-w-friends-come-hatch-learn-chat-meet-new-friends/33800_100#post_16390526
It seems recent studies have shown you probably don't want to increase the oxygen for the entire hatch.