if you absolutely want to maximize dps go ahead. but its not necessary i mean only the hardcore guilds are gonna force players to use dual specs for raids otherwise i don't think it matters.
So if you are in a hardcore raiding guild (the best guilds in the world, that down things before anyone) that wants you to have a secondary dps spec, because you can (gasp!) hearth and respec for a new tank spec, you aren't the main tank? Even though you are in a cutting edge guild, and very possibly one of the best players for your class? That makes absolutely zero sense.
Those that run true dual specs for the sake of their raid are the real main tanks. They are the players willing to step outside the box and make the hard choices so the raid succeeds. So, my healing and dps gear sets are way better than my tank gear. However when I'm tanking, the experience in all three roles will combine to make me a better tank than I could have ever become by simply staring at feet for 2 hours three nights a week.
Everyone is acting as if having two tanking spec is SO beneficial to the raid, it isn't. Sometimes, maybe, but overall having your tanks as dps dual spec will get more use.