I keep seeing mention that what keeps people from using a dps weapon to tank currently is the defense requirements. This is only part of the picture. There is no maximum survivability a tank needs. More is always better. The reason that a tank would not currently want to use a dps weapon to tank is because of the survivability stats that they would be giving up by not using a tanking weapon, regardless of whether or not you can easily maintain the crit immunity defense requirement. Stop saying that. It's a bad argument. Of course we can re-gem and re-enchant to make up for the defense. But you cannot make up the stamina or usual avoidance stat that a tanking weapon has. And even if you could with gems and enchants, what you give up to do so defeats the whole purpose, because you probably had to give something up just to fit that dps weapon in currently.
Another point, Blizz specifically hasn't put in a prevalent tanking weapon enchant on purpose. I don't see it as too far fetched to move to a druid / death knight design, by where you are balanced around using a dps weapon to tank.
And in terms of the continuous streamlining of stats and gear, mp5 and spirit is next. You can't stop homogenization. It WILL continue to happen. All we can hope to do is voice our concerns as it happens and try to give good enough feedback that we help sway the direction it takes.