First of all, there is no such thing as a meaningless game. While the second seed had already been secured, Pittsburgh's offense was in a funk (e.g. four turnovers in their previous game). Ergo, it was important that Ben & Company regain some composure and/or rhythm... and there are few better opponents against whom a team can regain their swagger than the Cleveland Browns. Excluding the injury, the planned worked to perfection: Pittsburgh's offense looked good against Cleveland.
Secondly, a few years back the Indianapolis Colts had secured their first seed with three weeks left to play. After they rested their starters for several games, it took their offense an entire half to regain their rhythm, resulting in an early exit from the playoffs. In other words, if the Steelers had rested Big Ben, it would have been three weeks between games for Ben. More importantly, since Ben is the type of player who needs all of the reps in practice in order to be effective, a lay-off of almost a month would have undoubtedly ended poorly.
Lastly, if Tomlin had indeed rested Ben, Steelers fans would have been screaming their heads off, citing all of the facts that I mentioned in the prior two paragraphs. "Ben is not playing!?! Doesn't he remember Indy!?! Tomlin has lost his mind!!!" Thus, no matter what Tomlin decided, he was bound to be ridiculed. Rain is wet. The sun rises in the east. And, Steelers fans complain.
Speaking of making tough decisions, after watching Norv Turner (San Diego's head coach) bench the injured LaDainian Tomlinson in favor of the healthier Darren Sproles, I hope that Mike Tomlin has the wherewithal to start the more effective Mewelde Moore. As I have always said, when he is healthy, I am Willie's biggest fan... but "when he is healthy" are the operative words.
In summation, being a head coach means making tough decisions. Sometimes, it means playing a starter, and at other times, it means benching a player who is ineffective. Thankfully for Mike Tomlin, he knows that no matter what he decides, Steelers fans will assuredly gripe that he made the wrong choice. Ergo, Tomlin can simply ignore the idle chatter from the peanut gallery, and make decisions based solely on his evaluations.