When you say that full defense spec has lower single threat generation, how much lower? Enough to not be able to main tank a boss? Or is possible to MT with that spec?
It's entirely possible to MT with a full defense spec. My Guardian is full defense because I didn't want to deal with the hassle of resource maintenance the hybrid has to deal with and have never had a problem tanking with similarly geared players. The first 15 seconds of a fight, where threat is most vulnerable from being ripped from you, are a bit riskier, but you can easily deal with that with some taunt spam.