I'm tempted to do this. I mean, I'm currently tanking Heroics in my DPS spec (Unholy) but I wear a combination of my Tanking and DPS gear, mostly to maintain 540 defense. Threat isn't an issue, and I still have Bone Shield and IBF to make up for any tense situations (these days it's almost always due to lag spikes or healer d/c). I figure the more DPS I can do while tanking through, the faster it'll be, and it'll make up for any softer hitting DPSers on the team.
Stay strong. Stay smart. All heart.