Some people have good ideas but not the technical skills, it would be shame for those ideas to never see the light of day because they didn't have years to spend learning the software.
The reverse can also be true.
In saying that I think most of the time the very best producers (who not always the most popular) are doing the bulk of the work themselves and it shows.
The reason I have not got into producing is because at this point I do not have the massive amounts of spare time required to do it well.
I don't see anything wrong with a producer doing the bulk of the work and then getting an engineer to tidy it up a bit as these can be two quite different skills.
Deadmau5 was an engineer turned producer and it shows, his tracks are always extremely well made although his ideas can often be lacking.
Sometimes two heads are just better than one!