While I've retired from my raid leading responsibilities, back in the day I probably would have "asked nicely" a few people to pickup dual specs, even going as far to have the guild pay for it. Of course the people that I asked would have likely been doing it on their own anyways. For instance I had an awesome Paladin tank who did just as good as DPS.
Right now the primary situation where dual specs are useful revolve around 10-man groups. Some fights are quite healing intensive and less of a DPS race. Those would benefit from having a hybrid switch from DPS to healing. An elemental shaman switching over to resto to heal the raid can make a major difference.
So how about it, is your raid leader making you dual spec?