I don't know if I'm getting old or what, but if you need to trail brake on the streets, you're riding too fast on the streets. If you're doing it just to keep the muscle memory/ keep your tool bag sharp for the unexpected, then I guess it's ok.
I think everyone should become proficient with it. @whaler saw it save my ass one time on the street when I went in hot and he saw my brake lights come on before disappearing up and over a hill/corner.
*not to turn this into a track discussion* Only reason I didn't cross the double yellow is because I felt comfortable enough to apply the brakes when and where I did. Only reason I was comfortable doing so was because of the few track days I had under my belt at that time.