Haha, so many great answers.
I dont understand why when people talk about litre bike, many of them link it to track.
Buying a litre bike doesn't mean one have to race it, or bring it to the track.
Let me tell you about my 10 year old ZX10R.
Stock, it did 160 km/h in first gear. Zero to license seizure on any public road in Ontario in 5 seconds or less from a standstill. In first gear.
The throttle is sensitive. Maybe 1 mm throttle opening gives you highway cruising speed. It is too sensitive for me to want to take it on a road course. Things happen VERY quickly when you open that throttle. If you take it to the track, it will eat tires and brake pads and scare the heck out of the rider. But that is what the bike is designed for.
I've modded it to take it to the drag strip because that's about the only place it makes sense (and then ran out of time and haven't had it out this year, but that's another story). Now, with shorter gearing and longer swingarm, it no longer does 160 km/h in first but acceleration is
violent. It can't be described any other way. It will also slide the rear tire out in corners on command of that sensitive throttle. Don't make a mistake.
You can't use it as a long distance touring bike - they're not comfortable compared to a more "standard" bike.
You can't use it in city traffic. The riding position is brutal without a headwind taking weight off your wrists. YES, if you are gentle with the throttle and feather the clutch, you can ride slowly. But the cooling system is not designed for city-traffic conditions. The radiator is small (in the interest of saving weight and bulk) and the cooling fan is not up to the task. So ... you can't get stuck in traffic.
Everything a liter bike is built to do, the police and the general public take a dim view of. Riding at high speed. Violent acceleration. Wheelies. Burnouts. Fun - Sure, but you have to make sure no one is looking ... or take it to a closed road course or a drag strip.
And the new ones are faster.
Smaller displacement bikes are easier to maneuver in traffic, don't heat up as much in traffic, a lot of them have much more rational riding positions for daily use, and that's on top of costing less to buy, to insure, to feed with tires and brake pads and gas, etc.