I don't know if there is a 'right' answer to this issue because there are so many ins and outs and details to consider. What now exactly constitutes an off spec item? I suppose a dedicated ret paladin would say a tanking piece was as would a rest shammy say the same for mail with AGI and AP. But would a piece with heavy block value and block rating be offspec to a protection warrior? Would a piece of cloth with hit rating be off spec to a holy priest? Would an item that favored a combat spec over a mutilate spec be the same for a rogue?
How many players in a raid are truly asked to ever go with their second spec? Maybe one or 2 hybrids are asked to heal. Maybe someone is asked to tank if another tank misses a raid.
I am personally against having different prices for the same items based on circumstances. Its alot of work for the person keeping the DKP. Its the same item no matter who loots it. I don't believe utilizing every single last piece of loot the most efficiently is all that important, either. If players don't want to spend DKP on it, they don't want it that bad.
your hat may be nice, but I have the little white tank top that says Legendary right across my boobs. I win. (or more correctly, H wins)