Well, I'm always Ret with Holy offspec. I always queue dungeons as both, then fulfill the role that it picks for me; I also only take loot for that role. This is for levelling purposes, quests will upgrade my DPS gear and dungeons will keep my healing gear on par. At level cap I spam with guildies and friends anyway since it always annoys me having people take gear I need for the spec I'm playing for their offspec.
It's not only DPS who do it anyways, I've had just as many Paladin tanks take healing gear from me, or roll on the 2h I'm trying to farm for Ret.