What I have found dual specs to be vey useful for are for being able to raid with friends no matter what our "main specs" happen to be. For example, friends that all rolled dps characters can play together just by having a couple switch to their tank or healing spec. It would be a bummer to not be able to play together just because we all happened to enjoy the same role. I personally am playing a healer, but most of my other friends have chosen dps roles.