They also "played it safe" (always a recipe for disaster in any context) in the last debate because they felt they had done well in the first 2 (especially the first) and wanted to ride that initial success to glory.
In all honesty KWRBT, the line is very fine because of the double standard. If Romney had gone 100% all-out, tooth and nail, he would have been very open to the charge of being a racist or "right-wing lunatic", which would have destroyed him. Look at what happened to Christine O' Donnell in 2010, George Allen in 2006, Libby Dole in 2008, etc. This is not 1988 anymore and the vast majority of the public has bought into the NWO lies.
Still, playing on the defensive is never a strategy to win. Just about any alternative to the way Romney ran in the last month (actually the whole campaign, but especially following the first debate when he stood a real chance) would have beaten how he actually did.