I've only been BBQ'ing for 2-3 years, and on occasion I did make my own rub based on others recipes, but I found it to be more expensive than buying some of the rubs already mentioned here.
Also, there's been a few rubs that had a unique flavor that I enjoyed, and by reading the ingredient label I've been mentally thinking of what I'd add to my own blend. So there's more than one good reason to use the commercial rubs.
Weber Kettle, UDS, Good-One Open Range
RK Coffee Roaster
KCBS & CBJ