One of my recurring concerns is with using a bicycle instead of a car as much as possible. In a recent post, I mentioned some of the problems: parking, carrying stuff, and not getting killed in traffic.
As far as parking, with a car, you can just stop someplace and get out. Your car's not likely to get stolen, and you can improve your odds by locking it, activating an alarm, etc.
A bike, on the other hand, is pretty defenseless. Even if you lock it, a bike thief can put it in the back of an SUV and drive off with it. You can lock it to a post or rack or something, but there isn't always one handy near where you want to put your bike. That's where the telescoping bike seat comes in.
All it takes is some telescoping tubing that can lock in the extended position. Not only does this make the bike impossible to ride (at least for non-contortionists), but it also makes it too big to fit in the back of most SUVs, etc. Of course, someone really determined could find a way to steal it, but who's going to want a bike that looks so stupid?
When you get back to your bike, you just unlock the tubing and collapse it to have a normal looking bike again. Of course, you might want to make sure the tube is locked down in the collapsed position. You don't want any surprises if you go over a bump.