It doesn't sound like a good idea. I'm sure you could get away with it most of the time, and definitely, in an emergency, I think it would be OK. The problem is, once in a while you get a lot of blowoff, or even a good vigorous fermentation may take it off the top of your fermenter. Those little S-trap airlocks are pretty cheap ( $1.69) and the ferm buckets come prepped for them.
EDIT: After refreshing the screen and seeing the previous post: I'm talking about beer. If you are asking the question in regards to mead, then I respectfully remove myself from this discussion.