I buy quilt kits all the time and I've quilted for a long time. I buy them from Fat Quarters and Fons and Porters and other online shops and my LQS. Also, at LQS I'll see a quilt on the wall that I love everything about it and they'll have a kit for it and I'll snatch it up. I also buy kits out of quilt magazines.
I don't see anything wrong with it. I take way too long when I try to put together a quilt kit myself. Usually the store owner takes over and by the time I'm done, what I have is what the store owner likes, not what I like.
I have lots of fabric lines that I makes quilts out of, but I have lots of kits in my storage unit and bookcase and drawers still. I'm not great with putting together a quilt, even after sewing for a long time. I have lots of doubt, so I love the kits over the LQS owners opinions.