Haas is in no way holding Shelton back from the 'big time' as you put it. Shelton was a solid midcarder, but he was unable to grow as a WWE performer. There is no doubt in my mind that he is the best pure athlete in WWE today and his wrestling ability has endless potential. However, his lack of spekaing ability on the mic hurts him greatly. He has no personality. Fans have no reason to 'care' about him. Like people have already said, the only way fans connect with him is through his high spots. If Shelton could cut a decent promo, then there's no doubt that he would be further up the ladder.
The WGTT were reunited because Shelton had done everything in the midcard on Raw and Haas was going nowhere, so they put them back together. Haas is also a very talented wrestler, but has the same defect as Shelton - no charisma.