That may be the point of the OP.  There are alot of reasons to do this and I really don't think there is anything at all wrong with it. 

Anyway it is possible that a person doesn't have the type of machine it takes to get the really good type of renders they want while modeling programs and test renders go just fine (I have this issue sometimes). 

It's possible like the OP said that they would just rather invest thier time and interest on other work. 

My only advice in this is to be clear up front and not to be analy picky with the person who does it for you.  Allow them at least a small bit of artistic license or its really not fun for them either.

I am:  aka Velocity3d