Here's a good article for you to read. This is from one of the first non-Japanese practitioners of Aikido. He discusses how his art has softened over the years;
Is Aikido a Martial Art?
I can tell you what I learned from stepping in a MMA ring; How to defend against a trained person punching me in the face while I'm in guard, or how to punch someone in the face while still maintaining side control or mount. It was highly beneficial to me because it allowed me to apply my grappling against a trained striker. That's pretty important if I found myself in an altercation and ended up on the ground with someone trying to punch me in the face.
Again, an Aikidoka doesn't need to enter a MMA tournament to get that kind of experience. They can just go to a local MMA gym and spar some people and test their Aikido against another trained fighter. How does your Aikido work against someone trying to punch or kick you in the face? How does your Aikido work against someone trying to tackle you to the ground? How does your Aikido work against someone clinching you and trying to take you down? How does your Aikido work if someone is on top of you raining blows down on your face?
Even if you get hammered, the knowledge you gain above is invaluable in my opinion, and it'll make your Aikido better. However on the flip side if you're successful, it'll make MMA better too. Which is why Aikidoka who shy away from such training truly baffle me, and leads me to think that they avoid such things for other reasons.
It's simple; If you can handle a trained fighter, then handling an untrained moron should be even easier. In my time in Bjj, I've sparred with college wrestlers, amateur boxers, MMA fighters, Judo and Bjj athletes, etc. After that, taking care of your fat, drunk uncle slapping around your aunt shouldn't be much of a problem.