the only reason to use holy ground is for when you want aggro. and yes you are threatening the raid when you use it like a damage ability. Sure it's so weak now that it's not a huge risk, but it's a risk all the same and not one I'm willing to take. I'm not interested in my parse number. I'm interested in doing my job, and risking the raid as little as possible doing it.
and yes, not alot of mobs memwipe. but for example, Sara in the Labs, requires a tank switch between Archtypes. thanks to all the hate abilities/armor procs our guard has, Holy Ground isn't good enough either. we literally had to put me in the MT group to amend our MT to take aggro, becuase 1 hate position and 3k threat isn't able to overcome the raid and his own threat on hit/threat when hit procs in the like 5-7 seconds I have to take aggro before the named 1 shots him.
the guard should not have to stop attacking, and tell the raid to stop attacking, so that I can do my job on memwiping or tank switch fights. This is what holy ground was for. to hit holy ground, and have aggro solidly no matter what for at least ten seconds...and usually meaning enough of your own hate is there so that when it wears off you are still tanking, until the next switch is needed or the mob memwipes/charms again.
we also used holy ground on mobs like Gynok, so that I would eat the DT and not the MT. again, a use that holy ground had, that doesn't work anymore. you know maybe is there was a level 90 version of Holy Ground that had double the threat numbers or something, maybe. but when all your big hitters have 10k+ spells, and everyone has 30%+ potency to go with it...3k hate and 1 position doesn't cut it.
our rangers/assassin and wizards and even our swashy post 15-20k+ parses. even with holy ground as it is, I'm not able to to overcome the threat transfer of the swash + Guard's 'when hit' threat procs + Raid dps + Guard's procs on damage. adn that's with our highest Ranger amended and the 5% threat increase to amend adornment...or the Assassin amended, depending on which one is there or in group. and lord help me if my Amend target dies for any reason, all my hate just died with him.