I am from all over, but my parents are from Idaho, so I'm not sure the regions identified in previous posts have a monopoly on the form. Anyway, I inherited "might could" from one of parents, and find it very useful. The way I use it, it deflates the less cooperative "might," alone as in "I might do that," which sounds like a teenager challenging an authority figure.

"I might could do that" suggests a willingness to try rather than an insouciant "I might," as in "if I feel like it."

I have a BA in English and an MA in teaching English as a Second Language. If one of my international students used "might could," I would be over-joyed. There are so many worse "infractions" with modals. Believe me, I see them every day.

I hear "all the sudden" in interviews on NPR. I agree with asheibar that such colloquialisms emerge because they are heard and passed on, but not seen in written text. It reminds me of the occasion someone had written on the blackboard, "It's a doggy dog world." Clearly, whoever wrote this had never seen the written version, "It's a dog eat dog world." Well, marzy dotes and dozey dotes and little lambsy divey" !

It does not bother me that colloquialisms emerge and colonize the language; what bothers me is that it seems that reading is becoming a quaint, anachronistic habit performed by backward looking people who haven't caught on that it's all in the tweet.