Personally I don't feel the need to do it at the moment. However, I can see several good reasons why people would want to do it for example now that the Federation has access to most KDF-races through diplomacy.
If it's just about people able to play a particular race, they already offer most of them via the C-Store. If making an officer into a PC offers more game mechanics than simply letting you play a different race, then it is game affecting and should not be an MT.
Paying for things like this is not an option to me. This sort of idea, and I think I recall seeing it mentioned as a future possibility somewhere, should be part of the game and funded via normal subscriptions. If there is to be a mega content patch released and charged for then I'd expect expansion levels of content which would be what we be on par with at least what we have available right now.
I think this could be a lot of fun, if they aren't removed from your crew permanently. The possibilities are significant. It would also be very 'Trekkie'. How many captains EVER led every single away team? How many first officers did something cool on the ship while the captain was away?
2-part missions, where you lead one away team and your second leads another. You play each part one after the other, and then rejoin for the final part (assuming there is one).
This could also be applied to those ships where the saucer splits off. Send each one through a different warp into a different part of the mission, and as soon as you're done with part 1 (probably playing the 1st officer with the saucer) you go onto part 2 with your captain and the main ship.
I'd certainly pay for this option, if they did it correctly.