It is definitely a good mon, I have used it myself(pre gen-7 to boot) to great effect, but it isn't good enough to be pushed back. The main reason is that after the first few gyms and starts to fall off a little (although gen-7 does help it). Another big reason is hardly a lot of people are going to use it, even though it works very well, as it is a rare encounter from a headbutt tree, of which there are few, as well as leveling it up being a bit of a pain.