I know of a guy with a 66 plate GT. He parks it on a main road in London and uses it like you would use any other car and has had no problems with it. He doesn't have a garage so has no choice.
I've always been a bit dubious about parking this sort of car just anywhere. Saying that, and as said above, I don't think Maseratis have the same sort of image problem that some other marques do.
That said, there is still a possibility and it is annoying to come back to a damaged car. My first car was keyed so I know how it feels albeit in the early 90s. Although, I've parked all manner of fast German stuff in all sorts of places over the years and not had any problems.
If I should buy a GT, I really am in two minds to just use it as a daily driver anyway, but still just use it like I would any other car. I think you can mitigate to an extent by parking away from other cars but there is always a risk. As davy83 states - it is hard work hiding it away.
As I've got older, I am finding it increasingly easier to be at ease about parking nice cars in various places. If you had a second car (which I have considered), you would have to consciously ensure you were in it whenever you needed to potentially park somewhere public.
Also the additional cost of a second car, unless you needed one anyway, would repair a fair amount of damage to a GT if it came to it - although you are insured anyway.