I really hate myself for this - I know language evolves, the OED records language usage, it's not the word police, etc, etc - but I really, really hate could/should/would of and therefore YABVU.
As pp have commented, I think it stems from mishearing the "..'ve " as "of" and then not entirely illogically extrapolating that mistake into writing. Easily corrected though, especially as "could/should/would of" makes no sense at all when written down.
I think the real problem is how do you correct another adult's English? It would quickly turn into a RP v regional dialect v snobbery v inverse snobbery bun fight.
It's a battle I don't pick. I just sigh inwardly and make a catty remark in my head about their lack of literacy. I know lots of lovely, bright and interesting people who do this so in the scheme of things, it's not a big deal.
Besides, I'm not the English police and I'd be cross if someone publicly corrected me (except in some work situations, but then I'd probably be embarrassed).
So the illiterates will win and could/should/would of will get in the OED 