I wouldn't agree that in order for the gameplay to be good the story needs to suffer, or vice versa. It happens to be sure, but I think that when it does it's more to do with the designers focusing on one aspect or the other and neglecting the rest. I wouldn't say that it
has to happen that way, just that sometimes it unfortunately does. A truly good game will be able to have an awesome story AND fun gameplay, and there are plenty of examples of those. Just from the top of my head I can name Baldur's Gate, Planescape Torment and Mass Effect but I'm sure there are more.
Now if we are talking about linear versus nonlinear storylines that is another matter altogether.
Also, How in the nine hells can SWOTR have a good story? How can any MMO have a good story? I'm not saying it has to be terrible but I just don't know how can you have a good story in a nonlinear, quest grinding MMO where singleplayer is a secondary aspect to the game and your character, the "hero" of the story in question, is just one guy in a sea of other players who should also be "the hero". How would that even work?