mostly because there's some confusion over what 'BiS' actually means.
if you see BiS in regards to a tank and think, that must mean the tankiest tank possible (maximum eHP) - allagan/myth accessories are BiS
if you see BiS and think, maximum possible stat totals - melded i70 things
more to the point, twintania has been downed a lot, by a whole lot of tanks wearing less than allagan/myth accessories.. so there's room to wear some dps/hybrid/whatever gear and speed the kill up
...that's not making you a 'tankier' tank though, BiS in that regard anyways, has always been the highest ilvl _of Fending stuff, and likely always will be
whether you're gearing for the highest possible eHP or something else is more of a personal decision though, hence the confusion
tldr - if your goal when gearing is to be harder to kill than anybody else, stay away from melded stuff, you only want the _of fending suffix
if your goal is something else, by all means buy some jewelry