Players can't take actions between the time you choose what to attach and the time they're attached to the target creature
just grab a friend, open two packs each, shuffle them together, and play
Up next is Thrasios, Triton Hero, one of the new cards in Commander 2016 that features the partner mechanic
It's not an additional cost
Even if thats you, youre still triggering your draw payoffs a bunch of times
For one thing, its one of only a handful of cards in Magic that says, Sacrifice any number of creatures. That effect itself can be quite powerful when you have Sacrifice payoffs in play, even if you dont get anything else out of it