The external fangs on a predator may be mostly for show. Perhaps at some point in their evolution they had a purpose, but for many generations they have not been functional. Although they can move them in threatening manners, it seems unlikely they can do so with a lot of power.
The way the external fangs (the fangs in the four corners of the predator's external mouth as it opens) are connected doesn't seem to allow for a lot of muscle. It appears, to me at least, to be more of a membrane that can close over something -- compare this to a human jaw, or the inner set of fangs on the predator. These are attached to large, solid bones which would allow for much more powerful muscles which are capable of biting with considerable strength.
I feel the fangs would not be useful -- predator anatomy (as I have seen it) does not seem to support them having a lot of strength (or, for that matter, fine motor control) of the outer fangs. While they might be able to cause them to protrude in an effort to combat the facehuggers, this seems highly unlikely to be able to fight them off.