Obviously Thunderfury is an avoidance loss over a newer tanking weapon, however I see some potential for it as a mitigation weapon. The proc can be triggered by hammer of the righteous, hitting secondary targets. This means that the attack speed debuff can be on multiple targets at a time, even without switching targets.
I suppose it would be possible to theorycraft this. Compare the avoidance and block loss to the proc chance, the chance that it will hit a new target, and the anit-haste (which has the added benefit of causing HS charges to not get burned through so fast). Unfortunately this is a bit beyond my abilities. Maybe I can finally get recount and make it work properly and do some comparisons.
Or I'll forget and just keep using TF for style points. On that subject, why are people so obsessed with stats that they'll freak out over a weird item here or there? Sure TF is inferior to a new tanking weapon, but is it enough to make a difference? It's like they say: "Good tanking comes from happy tanks, and happy tanks have toys."