Now you would fill it in with the address of the person you are asking to build the block.
The way it is now, it's whoever builds this gets it.
If we needed to, we could have a BitTorrent-esque tit-for-tat for transaction broadcast. Relay paying transactions to me, or I won't relay them to you. It probably won't be an actual problem though. It only takes one node relaying like it should to cancel out 7 others greedily not relaying.