Not trying to be smug, but if you've been paying attention, Thompson has been living outside the paint all series already. The Cavs have been switching screens 1-5 and Thompson has spent more time guarding the perimeter than actually being inside. In fact, the past two games, LeBron has been used more as a rim protector than Thompson has.
Speights hasn't played because last few games he was getting abused on the pick & roll. Yea he can stretch the defense, but right now, Cleveland is already switching everything and having some level of success with it, so Speights doesn't bring anything that the "death" lineup doesn't already do. I will grant you I think he's a better option than Varejao. However, the way he was rocked on switches the other day, I don't blame Kerr for being a bit concerned about putting him out there.