Rare? Not really.
Pain in the ass to get? Definitely.
While not rare per-se, it's not something you can acquire early when you first start a world.
It also takes considerable effort to get. (Without dropping it in the damn lava or getting your ass shot off your platform by a ghast.)
It should also be worth noting. 1 Lightstone block makes 1 Lightstone Dust. You need 9 Dust to recreate 1 block. So in this sense, if you wish to use it as a source of light, it is quite rare, or at least limited.
Why you would use it as light, is it produces Light level 15, the same as sunlight. I believe this will ignite Light sensitive monsters that touch it, the same as sunlight does. (Though, I have not confirmed this, so I am speculating at this point.) =3