Like I mentioned before and as other did as well, it makes him look more creepy. And because of that I think it works better. If he would have had a Lizard head, people would go like "Man, he looks totally badass!". But the deformed, humanoid head, gives him something unsettling. It drives the 'man turned monster vibe" home, much better than if he would be full on Lizard. And IMO, it works also better if you still want him intelligent and being able to talk.
I only really want him to turn completely, if all of Connor's humanity is gone and he only acts on animal instinct.