You know, the only major thing I can think of that I thought was wrong with the show is that most of the odd looking characters were pretty much people with make-up and masks. Other than that, I thought the show was good entertainment even though I was more of a Gilligan's Island type of person.
You pretty well nail the character to a tee. Probably better looking than the make-uped one!
its probably becuase back when it was made it was cheaper & easier to make more humanoid style aliens. plus if they are actual starfleet officers it is always easier for us to relate to a character who has a more humanoid form. i know in voyager we did see some aliens which were less human looking, i think that is because by that stage there was better technology to work with in regards to special effects which could lead to more 'alien' looking aliens.