I am the poster's friend who talked to him. As we have seen in many of the Cosmere's books, when vessels take a shard, they are changed, and lose sight of their humanity, their balance, and they lose control over their choices. Hoid knows that this power corrupts, and because he wants to achieve his goal, and taking on a shard will probably change his goal, he opted not to. It's also likely, based on how shards interact, that a vessel holding a shard wouldn't have the ability to hold other forms of investiture. This prevents a shard from being able to unite the shards. Also, in the traveler, he specifically said that bringing someone from the dead wasn't the only reason. Shards obviously have no way to bring people back from death, as we can see by the accounts of harmony, and the context from the traveler, so this realm seems to be inaccessible to any power currently in the world. Hoid's only hope to bring back people from the dead, may be reuniting Adonalsium.
