I find that Tim's comments on this are quite interesting. On one hand, he states that we should not play with the kits for fear of ruining them, then on the other hand, he states that if you use a kit on a second run (over your already pressed grapes/pommace, that you can make a better wine out of the second run!
I know Tim has more wine knowledge in his baby finger than most of us will ever know, but sometimes the "marketing monster" emerges, and he tows the company line. I can't blame him for that, but I always seriously consider what he says out of respect, then make my own decisions. In Canada, vinifera raisins can be purchased and are frequently used in kits here. Mostly the results you get from them are quite positive in lower end kits. I think in high end kits, the results are more subtle, but they still usually alter the kits for the better.