I suspect that it's easier with the "character actor" roles than the male lead type roles to accept new actors. It strikes me that Spock and McCoy are and always have been character actor roles. Quinto is likely to only ever play character actor roles (he just doesn't look mainstream). Nimoy is another actor who was never going to be a leading man. They're both fantasy fodder and outside of that, both had the choices to pretty much either play villains or in Quinto's case, side-character gay guys. And Urban has essentially gone from a minor LotR human (fantasy gear) to playing a role that Kelley did as a character actor kind of role.
This is probably why it was necessary that Spock and McCoy are played as more straight interpretations of already developed characters, while Pine couldn't take much from Shatner without it being a parody. Pine had to play Kirk without playing Shatner. Quinto and Urban wouldn't be penalized for taking from Nimoy and Kelley in that same way.
I agree that Quinto and Urban had far more leeway, and frankly, I think Urban did too much imitation of DeForest Kelly. He was very good, but he essentially replayed the same character. Quinto brought something different to Spock, in terms of more emotion, but it's still the same Spock I am familiar with from TOS. If Pine had imitated Shatner, it would have been death for the character, and he couldn't hide behind a stoic, calmly logical face, like Quinto. That must have been difficult for Pine.
What Pine faces, however, that Quinto and Urban won't, is that Pine might face the Mark Hamill problem. He will have an uphill battle to be respected as an actor because he is a shoo-in for pretty boy leading man roles. Granted, even the Mark Hamill problem is a better place to be than doing Princess Diaries II and Lindsey Lohan movies. He's definitely gotten a step-up there. But Pine doesn't have the benefit of playing a character actor role whose character actor can be impersonated without coming off as a parody.
I think Mark Hamill has gotten a raw deal over the years. After Star Wars ended, he went right into theater. He chose not to make more movies right away, and to instead return to the stage, and he was VERY successful. I think he spent five or more years acting on stage, and when he returned to Hollywood, they apparently had a lower opinion of "stage actors" as if they weren't "movie stars." People in Hollywood acted like he simply dropped out of acting for five plus years and never gave the respect deserved to stage actors. Harrison Ford went from one movie to the next, which launched his career, but I can't honestly say that Harrison Ford is a great actor. I'm not sure he would do well on stage at all.
Hamill deserves so much credit for making the Luke/Yoda scenes so memorable. Considering Hamill spent much of The Empire Strikes Back opposite a puppet, and yet sold every single scene with credibility and emotion is a testament to his talent. Okay, rant over.
That being said, I don't think Pine will have the Hamill problem. First, he has some solid performances under his belt (Smokin Aces and Bottleshock), and he is currently in a play that is giving him rave reviews. I'm sure he'll land a new movie role once the play ends (since his movie with Denzel Washington seems to have stalled). Pine is on quite a few producer/director's lists, so I'm sure he'll be fine. I certainly hope so because his performance was the one I enjoyed most in Star Trek, so I'm hoping to see him in something new soon.