Because Steady Shot is your #2 damage source that also procs your #3 damage source, which is from crits.
I guess that's an Icecrown luxury when you can just ignore all mechanics and zerg the boss in 40 seconds.
You don't Multishot anything in P3. You sit in Viper during rooms and get full mana every room. Your goal in first transition as Hunter is a) getting out proper Misdirects on Ragings, b) helping with spheres if needed, and most importantly c) getting full mana going into P2. Because P2 is the most important phase of LoD, you don't want to have to sit in Viper and eat a DPS loss; it's significantly easier to maintain mana than it is to regenerate it. The goal is to enter P2 with full mana and maintain it until first room in P3. Ideally, you want to be at 0 mana just as you're going into first room, because you can sit in Viper in the first room and regen 100% of mana. Your role as Hunter in room is not to clear spirits like Shadow Priests or Boomys, but to kill stragglers that are moving towards the raid.
The damage loss from Dancing is negligible. It's overwhelmingly more efficient at regaining and sustaining mana (and DPS) than swapping to Viper and firing off 4 Steadies. You get more mana back, in less globals, you don't suffer from the damage penalty of Viper and you don't have to sit in Viper for longer than a global.
https://www.youtube.com/watch?v=NlmjffMxkzg
Here's a vod of one of my streams in a guild I play in. 7/54/10 spec with no Rapid Recouperation.