Don't see the practicality of it at all, to be honest... except in a car.
Bad guy walks up.
"Hey how's about you give me all your money?"
And points a gun @ you. What are you going to do/say to get to your ankle rig?
"Uhm, hold on buddy let me tie my shoes first."
I figure maybe for a last ditch weapon for a situation where you end up on the ground fighting the guy, an ankle holster
could come in handy, but even then IMO that's a long shot. After all, if you end up on your back, you've already messed up.
I wouldn't do it, and I expect to be paid for my advice, so you're out $.02
But seriously, if you're comfortable doing it then go ahead... At the very least it's one more option, and options are never a bad thing.