I think if you get like a J or a K, a red flag may go up, but if you have a decent verbal score, I doubt it is a big deal, especially if you fall in the middle tier of writing scores. I know some/most adcoms have been noted as saying they don't even screen for writing scores, and it's hardly come up interview-wise. I think the gain from receiving an S or a T is less than the loss you may get from receiving a J or a K. I think they will see your essays as a better indication of your writing.
I could be wrong though. I mean, I guess it's on there for something. But that's just what I have heard.