I do this all the time too, and am surprised more people don't do this. Is it not secure enough for your tastes?
To me, locking the helmet prevents casual theft. If someone really wants your helmet, they're going to take it. Hell, just cut it off at the D-ring and all these expensive locks are rendered useless. The helmet is useless too, but someone who's going to steal a helmet in the first place might not be a logical enough individual to put two and two together.