I will throw in my 2 cents and agree with Adam. For a fun run, do whatever you want. If you are joining a Kara run expecting to down Prince, you need to get the highest DPS build possible.
As Adam mentioned, I have tried out the different builds in Kara - MM/SV, SV/MM, and of course the BM/MM build. There are those who argue for the MM heavy builds and their massive dps, but those must not be combining their pet into their dps figures. That "massive" dps is a fable, carried over from pre-TBC days. It just Does....Not....Happen anymore. At the highest gear levels, MM has the LEAST dps of any spec. SV passes MM at about 800 agility, as RAP just doesn't scale as well as you would think, where Agility multiplies with itself. I can certainly understand someone who enjoys this build as it takes very little pet management but it must be with an understanding they are giving up dps.
The same is true with SV heavy. Until you can hit about 700-750 UNBUFFED AND PRE SV agility (which would bounce you to near 800 unbuffed agility AFTER SV), this is not a viable DPS spec compared to BM. If you have that sort of agility, your overgeared for Kara anyway, so spec what you want. I very much WANT to be SV, but it ain't happening anytime soon. I know the argument for better trapping etc. If you are running a lot of heroics with a group that understands you purposely gimped your dps for better group utility, fine, its a great spec. But that particular utility has very little benefit for a Kara party. You can try to justify it, but it isn't going to fly. Traps simply are not needed in Kara. Many groups do without ANY hunters. If you have 2 pieces of D3 gear, you are set for Kara. Just swap them out as needed.
Most groups I have run Kara with outside of our TF Group 1 are absolutely stunned with hunter dps potential after the run. I get a LOT of tells, especially from those who have 'Never' had a hunter beat them in raids. I was in one group where we took down Prince, among other bosses of course, where I was 5% above the nearest dps'er (I had about 25% of the entire groups dps). Most people vastly underestimate the potential of the hunter in their group. This is not a brag post. The reason Group 1 ran with 3 hunters for so long is because it was EFFECTIVE, and NOT for their trapping. Beyond Moroes, we just didn't trap much. It was all about the DPS. My best dps right now hovers around 1350 or so on some boss fights. More normal hovers btn 800 and 1100 depending on how much I have to move around.
In conclusion, feel free to spec whatever you want. Its a game. Have fun, or its not worth doing. Just be aware of what decisions you are making and how they affect the raid. If you make a conscious choice to limit your dps due to enjoyment of another spec, make sure the group knows that, as you are potentially affecting the rest of the groups enjoyment by limiting how far the group can progress. I get the most fun (and a LOT of Kara invites) doing the max damage I can pull off. Your mileage may vary.