Can someone explain to me Lazy Tanking please.

[Reinfeld]

I would just like to chime in here for one second. As someone who had played both healer and tank extensively I know you technically do more damage without Defiance, but if your healers are doing "other things" as you said (by which I assume you mean they are DPSing) it's likely that Defiance will allow your healer time to deal much more damage than you could contribute in your unbuffed state. Defiance really makes that big a difference between your bigger HP pool and buffed heals. That's especially true when you reach level 40+ dungeons and your WHM/SCHs have access to Holy/Bane.
Now, if I get one of those lazy "I'm gonna sit here doing nothing for 10 seconds then throw out a cure, regen and afk again" sometime's I'll say fuck em and drop Defiance.

And the term "Lazy Tanking" isn't in common use so I have no idea what he meant by that, I would have had to see what the run was like. Though if you have your warrior crystal you should never be running Marauder. The Devs intended for classes to be "good for small party content", but in reality they are complete and utter garbage.