Which is fair enough - but would your general movie-going audience laugh at a Megatron gun? That's the question Bay & co had to ask themselves.
It goes beyond adult transfans, Dreamworks/Paramount cared about adult Joe Public. So mass shifting is out. And Megatron needs to be able to use his alt mode in a one-on-one with Prime.
Personally, I prefer Megatron as a tank, and I don't think much of Nerf Megatron. I'm not in favour of the jet, but I can see why the movie makers went that way. The fact is it makes him more of a menace with a flight capability.