Very interesting question. But what you found seems to be opposite to previous findings. For example, the figure from a previous paper (Fig. 14 of this paper) shows that including T2 data would get thinner (and better) measures. That is, the red line shows the pial surface with only T1 data, while cyan line shows the results based on both T1 and T2. Actually, it is expected that including T2 images would clean some blood vessels and dura close the gray matter. 

Hope this helps. 

Xiangzhen