I believed that the point of touching my knee down was knowing when I had reached maximum lean angle, ie, just before some part of the motorcycle is about to deck out. I also believed that knee-down would only make sense on a sportsbike, and in practice, only on the racetrack. I must confess that I don't get my knee down when riding my not-so-fashionable motorcycle on public roads. A few times my toe sliders just touch the ground, but that is usually due to the positioning of my foot on the peg.

I do actively ride the motorcycle rather than resting my arse on it – thighs and knees usually testifying of a decent ride. Great fun. In spite of the lack of knee contact with terra firma. I'm fairly relaxed about this, but have I lost the point somewhere?

If not, why stress about 'getting your knee down', especially outside the race track?

The point of putting the knee down is body positioning. You also want to point your head to the inside of where your turning. Basically you try and stick your head past where your inside mirror would be(that is if you still have one attached) while still looking forward. Same direction as where your knee is pointing. Basically you are countering balancing the bike to leave more of a contact surface (your tires) on the pavement. If your body is hanging off of the bike one direction, then your are allowing the bike to make the same turn while actually leaning less (thus leaving more of a contact patch on the pavement for the tires).
Hopefully that makes sense. What you don't want to do is put a knee down while your head is pointing the opposite direction. Your spine gets twisted up and screws up the balance. There are some good vids on youtube describing the technique Also the knee isn't the maximum lean angle. Riders in MotoGP will put an elbow down while having the bike at about a 64 degree lean angle.