It certainly IS a bitch to remove, but the process does not scratch the glass - at least speaking for myself anyway as I've removed the tint on a previous car of mine...
It was at least 5 years on and it was well and truly 'fused' to the glass, but slowly and painstakingly I removed it using mainly razor blades and with the final result there were no scratches...
People tint their cars as Miz as stated mainly for looks (each to their own) but the obvious benifit is reduced heat irritation to both you and the cars interior, especially with the lighter or higher percentage light transfer rate tints...
I've got 50% on my vert, it looks heaps better than darker legal limit tint and reflects more heat as well - personally I can't see the sense in going to the darkest possible tint shade unless you want that look, the darker the tint, the more heat absorbtion you get, but as said each to their own...
As for factory tint, that's all a big load of BS !
Unless the manufacturer actually applies a film to the glass, (like some Mercs have) it's not factory tinted !
Factory tint is just a marketing tool used by the car manufacturers, but what it really is - is the natural finish of the glass, (greenish colour) which in this case is called 'float glass' and is caused by the amount of lead and other metal contents in the glass molecular structure...
If you get crystal clear glass, it's known as 'opti' or 'low iron' glass and has almost zero lead/metal content, it has no greenish colouration at all but is much more expensive and the car industry dosn't use it...
I know this from when I worked at a mechanical engineering firm who had large dealings with the Pilkington Glass company