i havent taken cover into account but how long is cover going on for these days? i dunno if it last that long so it can make a huge difference , also if mobs dont have aoe attacks i can see this being extremly abusive as well, altough the tank would still be doing its job that is dont letting other ppl getting hit, it can´t be a perma tank (by perma tank i mean without chance of losing in the hate race to anyone), now if mobs do have aoes the mages would be putting thenselfs to danger (save covered ppl) to have the enmity down effect so i see no issue in that specific scenario (also looking on the bright side of it would be nice to see more ppl using cover), anyway i belive that can be solved just by crunching the numbers anyway.
Lets get some fictional numbers runing for a bit, lets consider a hit taking down 10 enmity and a 100 enmity cap, hiting the mob with an auto attack generates between 1(dagger/swords)-3(sycthe/GA/GK) enmity , ws´s 4-6 and magic 1-15. now lets say the JT makes everyone around the tank lose 10 enmity (100% as if they are taking the hit thenselfs) and at max level trait the tank losses 7 enmity ( 30% less) the diference would not be big but enough for the mob target the tank but if the mages go with a median-powerfull spell or the DD´s with a median-powerfull ws they may be targeted instead, however if the tank manages to pull hate before that happens the mob will probably stay on him for one more swing reducing the enmity again.