Because it is possible to develop bad habits and suboptimal techniques in that case. And this gives me an idea... why don't I take you on as something of a student? You'll get out on the adventures and gain the benefit of training in a broader range of weaponry, some of which might prove more efficient or effective against the creatures out there than your current weapons, and I'll feel confident in bringing you knowing that we'll be honing your combat techniques.

Plus there are things you can teach me in turn, I have no doubt about that.