i dont use boat bags anymore but i can still see some slalom boat owners use them. and i think it's a good idea especially if your boat will go inside ur car after kayaking..
Its not that necessary but it should be easy to stock up your kayak in your car. :) I use a boat bag still just to keep it protected and intact. Most of my friends find it silly, but I beg to disagree. I'm very meticulous with my stuff especially the important ones.