I've always seen Ben as the one who believes in what the FF do more than anyone else. The FF do change the world, and they do it as much as Thor or Cap does. As we saw in the Thing's recent series, he's also a leader in his own community, trying to make Yancy Street better in a number of ways, one of which was to keep its kids safe by giving them a youth center to go to instead of killing each other in gangs like his brother was killed. Isn't that basically the same as Thor running around killing giants to keep Asgard safe, as far as how high the goal is, respectively? He was still worthy of Mjolnir when that was pretty much all he was doing, remember?
The current set-up of the FF kind of goes against the "loyal right-hand man" idea, too. Ben was very much taking the lead at the beginning of the FF's last mission. Let's not forget, Ben worked his way through the Air Force ranks to earn one of the highly coveted spots in the astronaut program. Leadership skills are high among the qualities they look for in that program. Granted, Black Panther is really the one who started coming up with ways to try and defeat the Surfer on the FF mission, but a good leader knows when to yield the stage to those who are better suited to solving the problem at hand.
As for his self-esteem issues, he still has them occasionally, but he's consistently persevered and, perhaps, shown more heroism in his choices there than anywhere else. He's had the opportunity to return to normal before, but he's always chosen to shoulder the burden of being the Thing for the greater good. If that doesn't count as self-sacrifice and nobility, I don't know what does.
Plus, if you're gonna get into selfish aims, let's take a look at Beta Ray Bill. His only aim when he lifted Mjolnir was to kill Thor so he could keep his people safe. He wasn't aiming for any grand, high purpose beyond his own people's survival. He also had a lot of those same self-esteem issues Ben had. He was an outcast among his own people because of his enhanced cyborg body--which he only got to protect them. That's why he was so happy in Asgard. Suddenly he found himself in a place where people accepted and respected him rather than viewed him as a freak, and he even found a woman who could love him in Sif.