We make all of our rubs. We started with commercial rubs and then started modifying them. The problem was that you don't know the ingredients and proportions in the commercial, making it very hard to make changes to the rub that give you the change you want. Even small things like "it is a little too salty", your choices are to add something to try to offset the salt or look for a different rub. You can add but you can't subtract. Another thing is the quality and freshness of the spices. Papricka for instance. Hungarian doesn't darken under heat like Californian or Spanish. You can have raw sugar in the rub rather than processed sugar. You can have course salt rather than table salt. And we order spices for each contest so we know they are fresh. Also, knowing the rub helps in making a glaze or mop to go with it.
I've really liked a commercial rub and had the company change the formula. A lot of these really great rubs come and go from the market.
All of this said, do we have the best rubs out there? NO. But they are unique. We can modify them. And don't have to worry about our rub disapearing from the market. The downside is it takes a lot of time and experimentation to get a good balanced rub and then come up with new ones.
There are several good books like Smoke And Spice that have good basic rub recipes and discussion about making rubs. Once you start, I bet you will get hooked.
Good Luck