Jeffa,
Let me get this straight, what you are saying is that you used the serrations on your partially serrated knife so much that that portion of the blade is now dull -- Okay, that makes sense.
The manufacturer won't sharpen the serrated portion, so that portion of the blade is now useless -- makes sense.
Since a large portion of the blade is useless, the knife is of little use -- yes, makes sense.
For this reason, one should not buy a partially serrated blade -- no, I do not agree:
I think you are ignoring the use you got out of the serrated blade you dulled. I don't know what you were doing with it, but whatever it was I doubt you would have been able to do the same thing with a plain edge. What I am saying is that you are ignoring benefits you received from the serrated edge and focusing on the negative aspect (that being the rapid loss of edge and difficulty of sharpennning).
IMHO, it is obvious that the serrated portion of a knife is not there to be used continuously. It is not a "work" portion of the knife. As I see it, if you need to use serrations continuously for a job, either get a buck saw or dull your knife serrations. It is for this reason that I would propose that the serrated portion of a knife blade is for occasional emergency use only. If a knife buyer is not interested is such an application of his or her knife, it seems clear that they should get a plain edge.
Excuse me, I must cut down a giant redwood with my partially serrated folder now.