I think that SH appears so much because she carries a very important item for the story (she also appears a 3rd time, at the goblin camp and when getting close to the githyanki). I think that beside hers the other companion that appears later on is Lae'zel (in fact, if you rescue her and don't team up with her she's the only one that moves one).
The rest of companions don't move at all. As far as I know...
I know SH carries a important item but with the amount they shove her down your throat, it makes the other companions look complete idiots.
Also, Who in there right mind groups up with someone that just tricked them and then holds a knife to your throat? It doesn't make sense to recruit Astarion, the only reason to recruit is because he has the same problem as you and with how early you can find how his secret killing him is the most logical choice at that point.
