Bad idea because it's a better design to use a 20 amp circuit for outside outlets.
I don't agree with Jim's opinion that double-protecting a receptacle (AFCI on top of GFCI) will cause problems with people wandering around in confusion when something trips. But to each our own.
Um... No matter how easy we try to arrange things, don't many people do that anyhow? I know you've run more than just a few service calls to reset a breaker or a GFCI. That said, doing an outdoor GFCI on the AFCI circuit is just plain silly. On customs, I generally try to rope all the outdoor receptacles on their own dedicated circuit or circuits.