This is what we do too, though we have the added clause of "unless it is an actual upgrade for your current job" to the i180 or less. (Ran as my secondary role of scholar today, and while I'm a high enough ilvl to enter, I still have one or two i170 pieces, plus one stupid i160 bracelet ^^; )
I... don't see why they couldn't have done it the way they do for hunt marks. If you're a healer, for example, you still need to remember to hit the hunt mark at least once, or it doesn't matter how much you heal people; you'll still get nothing.
Of course, hunt seals/tomestones are quite different from treasure coffers, so I understand it wouldn't have been as easy as it might seem at first glance. But surely it would have solved the whole 'run off and do MY thing, but still roll on YOUR loot' thing.