I don't like Amazon's attempt to tie you in to only their format and now this restriction on which programs are authorised to produce MOBIs. I'm sure Kindlegen is very good but Amazon seem to be excluding other generators, not just recommending.
I really appreciate the open nature of ePub and the ease with which it can be tweaked. I can buy ePubs from many different publishers without being tied in to any particular one.
If Kindles supported ePub I would buy one without hesitation.