We've all been near a big Harley at a stop light, where the rider keeps blipping the throttle. I've always wondered why they do this. Is it because those big pistons don't idle very well, and if you don't blip the throttle they tend to stall? Or is it just a bit of macho (or occasionally macha) fun?
Mechanically, it's a bit of a throwback to carburettor equipped bikes - my motorbike, for example, can be a little bit unforgiving when it's cold and it's not nice to go to pull away and have the engine splutter. In fact, an inexperienced rider could easily fall off because of it! And, as you've highlighted, a badly set up bike might have issues idling.
That said, the truth is that most motorcyclists love the sound of their bike. We also don't have much else to do while waiting at lots - a drive might fiddle with their stereo, some bikers blip the throttle.