Allison: I heard they were getting rid of ArmPen in Cata. But, let's assume there is some other stat that forces all Melee to prefer Leather. I'm not sure that's plausible from all the announced changes so far, but I guess it's what you're stuck on.

You say: "it's dumb not to want the raid kitted in their best." Note, that you say, "the raid" and not "myself". So, right there, it often makes sense to force people to prefer their own armor type or the raid as a whole will have trouble getting to "their best." Or, just farm forever and never actually attempt hLK25 in an effort to fill out everyone's BIS list.

But, you're talking about off-spec. A couple of options present themselves. Go Boomkin! It should be way more shared gearing between the two specs (no +hit, Spi desirable). No, you say? Go to the previous Tier (basically the same as going to 10 man now) which for Tier 11 will be lvl 85 Heroics, so in fact it will be EASIER to gear your off-spec. Beyond Tier 11, not much can be said because that is just way too much speculation.

Anyways, I don't think it makes a lot of sense to put hLK in the same argument as gearing your off-spec.