Thrall's intentions may be more far sighted than we realize. Thrall knows perfectly well that Garrosh is not fully up to leading yet, and is counting on the other Horde leaders to keep him in check until he grows into his role. The goblins are a potential spanner in the works, though. If Thrall had a new leader appointed in Gallywix's place, the odds are high that new leader is someone who would feel grateful to the Horde for its aid. Grateful enough, perhaps, to not question Garrosh or work against him if need be. Nor can Thrall tell his new member nation "Welcome to the Horde, don't turn your back on the guy in charge." But with Gallywix, Thrall has someone who will do exactly that without Thrall having do anything to ensure it. Gallywix can keep Garrosh on his toes in a way no other Horde leader except maybe Sylvanas could, but with the goblins as a whole being a lesser risk than the Forsaken.
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Quoted from a debate on tvtropes. I think this has some pretty good basis if you really think about it.