When you take any money at all from a big VC in a seed round, you are effectively giving them an option on the next round, even though that option isn’t contractual.
via cdixon.org
Great explanation of the issue here. It's a potentially lose-lose situation. If the VC wants in, they can collude/drive down the price of your Series A. If they don't, then you smell bad and can't raise again.
Multiple people in the know advised us to steer clear of this option while we were raising.