1) Why? Ignoring any technical aspect, I feel far more comfortable with the prospect of Valve behind it rather than Facebook. Valve is essentially the PC gaming market, I'm not too worried about them or their support going anywhere, games are their business.
Games aren't Facebook's main venture, it's something they bought in to. It's an asset that while they are pouring money into it for now, how long will that continue and especially if returns are not what they need to be?
TLDNR: I'm far less worried about long term viability through HTC/Valve than Oculus/Facebook.
2) Ditto what @Troll said, no need to really buy anything else for flight sims.
also as i didn't see it mentioned in here before touch type controllers are on their way for the Vive