Why wouldn't you.
Guns are an everyday thing and can present a real problem if you are forced to deal with one. Disarming techiques should be practiced as needed. Guns are around you all the time. I am even wearing one on my hip as I type this. (I'm a Policeman) When I was in the police academy we were taught afew simple disarming techiques and told "You will probably get shot doing something as silly as taking gun from somebody. If there is a 100% chance somebody is going to shoot you, why not do something that reduces the chance of you getting plugged to 75%" Virtually all Aikidoka have more hand to hand training than the vast majority of policeman. (Cops have the advantage of the occasional randori with unwilling Uke/ a humbling and enlightening experience) I went to a ground fighting school once and we spent three days wrestling around and striping guns from each other. The techiques are easy if you don't have to think them up on the spot.