I have a Priest dual-specced Disc and I had my first actual healing stint since Wailing Caverns back in the day. I healed heroic Old Kingdom and it went pretty well. The only actual non-boss mechanic (Insanity ftl) and non-bug (bugged Jedoga adds) wipe was when the tank tanked a pack of those @#@! elementals down the stairs and the ranged DPS was up above the top step... which meant I had to run back and forth to heal either the tank and melee, or the ranged since the LOS split my heals. At one point the ranged took a flamestrike, and the tank got banged up while I was running back to heal the ranged, then I took a flamestrike, and it went South from there.

I think dual-specs can be an overall plus, if for no other reason than some players will experience roles outside of their normal tracks and can use that to help them in their other roles. I learned a lot about healing and tanking while sitting back and DPSing. I DPSed as Shadow all through BC and balanced my threat on the 120% razor's edge. I learned a lot about tanking, since I had to know how the tank was generating threat so that I wouldn't pass them and pull aggro. I was doing over 1k DPS in ZA before the nerf without pulling through, back when number like that meant something. :^) These days when I tank on my DK or Druid, those threat lessons come in useful. I also work as emergency heals when I'm DPSing on my Spriest. I've kept more than one group up when the healer got aced in BC and in WotLK, which keeps me somewhat sharp for Disc healing. The only thing is I don't yet have the reaction time I should have when healing on Disc. I have to think about which spell to use and then find it on my bar... so I sometimes lag a bit between when I should throw my heal and when it goes off, however that'll tighten up with practice.