I agree that frame-mount locks are very convenient, but probably best for bikes too heavy for a casual thief to lift and walk away with. Almost every Indian-made DL-1 copy had one when I was a wee lad, all stamped from sheet metal, and all with, probably, the same stamped-steel key.
I last used one on a very nice short-distance grocery getter, a hot-rodded (*) Worksman tricycle -- no one was going to casually heft that over his shoulder and walk away! But it was very liberating to ride up to the store, flip the lever, no need for a bike rack because the vehicle stood up by itself, and simply walk into the store like any other normal human being.
(* Imposed a Stronglight 99 crank on it by beating -- very nicely, I may add -- an indentation in the "chainstay" so the crank arm would clear, so I could install a 40-something ring in place of the absurdly small 32 or whatever the stock was, for a ~67-70" gear; clamp-on combined bottle dynamo/LED headlight, Wald Newsboy basket, and of course a decent saddle; B 68, I think. Trikes -- I also owned a refurbished Ken Rogers BRT with huge Hoss saddlebag -- are the ultimately good grocery haulers.)