My first exposure to graffiti was consistent tags on a fence that runs in front of my driveway back home. This gave me quite a socially conservative view of graffiti when I use young because it looked dreadful and ruined a nice-looking fence.
While this is certainly petty on my part, it points to a problem of graffiti being an art form. It is often imposed upon people, who are rarely asked if they'd like their fence or train made a platform for someone's art.
This doesn’t mean it isn't art though - Banksy never asks for permission but few would doubt his work was art. If we need any proof, look at how much the pieces go for when they're auctioned off by councils! Irony of Ironies I know, given how much most councils fight graffiti artists.
So, permission to paint is never, and has never, been 'the point'. Instead I always fall back on two questions. First is the thing about self-expression? Second, does it offer a way to understand that self-expression to a reasonable number of people?
you never know where modern culture is going to take us
This is a pretty wide scope and includes a lot of crap modern art but it is useful in the context of graffiti. Is it showing something beautiful that anyone can enjoy (Think Berlin Metro's ingeniously graphitised Metros) or something meaningful to give most observers pause (Banksy’s stuff on the surveillance state). If the graffiti is meant to offend or intimidate it is equivalent to broken glass on the street - maybe fun for a couple of idiots to make but meaningless and a grind for anyone else.
Importantly, art either reveals something to the creator or audience which can't be verbalised or spreads some message or theme to a wider audience, no one would have heard of Guernica without Picasso's painting of the same name.
I'm sceptical of a lot of graffiti's ability to carry out these tasks, but after some of the shit I've seen in the Baltic, you never know where modern culture is going to take us.