I'll do this examples trying to explain my point of view on thread meter:
case 1: average-good tank + the spammers.
This will be short: spammers are a scourge. They should't spam at all. If they die, they deserve it, aggro meter or not.I don't think a spammer would bother watching it thou.

Case 2: less-than average tank + normal players.
Ok, we have a good pt but the tank it's a little sucky. Here's the point:
-tank do it's best, but since isn't so good, dds catch his hate often
-dds see the icon and stop damaging
-what happen now ? Nobody doing damage, aka the fight goes longer
-Mob isn't standing still, he's still attacking.Aka, the healer still heal
-On long run, the general efficenty of party goes bad
-eventually, healer run out mp, or nm rage
-guess what ?
In a environement like this, how can help you aggro icon? He don't teach tank do to his job better.He can save your life 1,2 times, but there are side effects on stopping doing your job.
This is my opinion.