Got to disagree with you there, a scraper will shift thick limescale to a degree, but it won't make hardly a dent in limescale that is etched onto the glass.
If it is very fresh limescale then yes, a scraper will get it off.
During the summer months I have to contend with it from hanging baskets over pub windows and so on, the drips from the watering systems get on the glass, and limescale starts to build up.
I scrape it off no problem at all.
At home I've a couple of windows that are badly stained with limescale, and you won't get close to it with a scraper, cillit bang won't touch it and MDR isn't much better.
I've generally told customers there is nothing you can do if it won't shift with a scaper...by the time you've messed about with chemicals and whatnot you could have cleaned another couple of houses!
Ian