We talked about changing it, but decided we liked it the way it is. It’s a pretty nice reward to suddenly be able to equip mail or plate at higher level. By similar logic, you could argue that we should make helms, shoulders, trinkets and even mounts at low level because they’re awesome upgrades and players are going to get them anyway.

It doesn’t actually make the itemization simpler to add plate at lower levels. We have to create more items and make sure they are well distributed. That’s not a huge consideration either way, but you listed it as a benefit and we don’t see it like that.

We also don’t think it’s a huge balance problem either way. Nobody is so weak below level 40 that perfect gear optimization is really going to make a difference between success and failure.

The strongest justification for letting e.g. warriors use plate at level 1 is the feeling at level 40 of having to swap out all of your gear. But even in that case if you hold on to the mail for a few more levels until you get a more suitable quest reward you’re not really going to be at a noticeable disadvantage. (Source)