I always saw it as X used his "alive" physical body as a way to stay connected with the real world...
but anyway, his power generator powers Yggdrasil doesn't it? with all that power of his being spread throught the complex, youd think X could somehow still recieve some power for himself, as in my opinion, since his physical body is still alive, he is still connected to it somehow. similar to how Zero and Omega are inevitably drawn towards each other, due to metaphorically being "two halves to a whole" Zero's consciousness calls out to its body, and Omega's Zero body calls out to Zero's consciousness. I think a similar relation can exist with X and his body. His body is useless, as it can no longer carry X's consciousness, but still is "alive". And thus, X and his body are inevitable "connected" in some incomprehensible form. as a matter of fact, it wouldn't surprise me if X's spirit hadn't haunted Yggdrasil for a while.
although, WAS he haunting Zero's chamber, or did he simply react to Zero being awakened and go see what happened?
also, while were on the subject, ill just leave this here.