I have the PMAG 21s on my vest for warrants because "why not?" I can't find a reason, they've been reliable and durable for the last 3 years I've owned them, ergonomics are fine and the weight penalty is negligible.
I'm usually just carrying a single spare 15 round mag when doing regular plainclothes work though, as you noted it's statistically sufficient and I found the 15 round is just *that* much easier to conceal universally, even though I can dress around double PMAG 21s if I wanted.
Extended MecGars were very common in my office when we still had SIG P229s.