I've tried tipping plastics with a piece of crawler a number of times without very positive results. The one exception was fishing for river smallies where I added half a crawler to the hook on a tube jig. This was to make a live bait, bigger version of the berkley atomic teaser which I've had very good luck with for crappie. The tube + crawler combo definitely out performed tube only but I'm not sure it did better than crawler only would have done on that day. I haven't tried it again, actually I forgot all about it until this thread.
Also, I know many spring coho fisherman swear by tipping a grub on 1/4oz jighead with piece of squid. Lastly, I've had really good luck in early spring fishing small spoons, where I have replaced the treble hook with single hooks and tipped with wax worms.