Queens Quay may be packed at certain times in the summer, but what about the other 80% of the time?
I previously lived in Cabbagetown and, before that, Leslieville. I have friends in Parkdale. Queens Quay would certainly be the best way for me to get from my place to theirs.
I've had two jobs on Queens Quay. Plenty of people commuted by bike at each.
Cyclists should bike in a safe manner that accounts for conditions, in all cases. It bugs me that even when things aren't jammed with people there's still an idea that this bike infrastructure is recreational, not functional.
The Lake Shore path you suggest would be a very welcome addition to the bike network, too.