I see your point, minion, and I understand, but I respectfully disagree. All the other times we've seen brainsucked-Tara act in a completely unresponsive manner toward anyone else, Willow has always been present. In "Tough Love" she freaked out and was reaching out for Willow as they wheeled her away; similarly, in TWOTW she was reaching for Willow as she went into the bedroom. Granted, her focus is almost entirely on Willow in this state.
But what happens when a good amount of time passes, and Willow hasn't come back? We don't know how she acted in the psych ward that first night, and it's never really clear how much time passes between Willow going into the bedroom and this scene. (Granted, not hours or anything, but it could have been anywhere from a few seconds to fifteen or twenty minutes.)
I think it'd be natural for her to seek comfort--not because just anyone can do so, but because she is very much aware of Willow's absence and worried by it. That's my read on it: I found it extremely touching because to me, Tara's agitation said a lot about her concern for Willow, as well as an instinctive desire to be with her.
At work recently, I found myself looking after a mentally handicapped young man who'd gotten separated from his mother. It was intermission at the theater, and she'd left him to use the handicapped restroom while she went to the women's room, probably not anticipating that the line would be so long. For the first ten minutes or so he was fine; while his mother was there, he focused on her, and kept to himself after she left, not really aware, it seemed, of anyone else's presence. But when he'd been waiting for a while and she wasn't back yet, he began reaching out to the nearest person, who happened to be me--he held on to my arm, followed me around, spoke to me, but the whole time his concern was for his mother's whereabouts. He kept asking me where she was, if she was all right, for me to make sure she hadn't left him.
So no, I don't feel it would have undermined anything as far as the strength of the W/T relationship goes. Your mileage may vary, of course; as I said I completely understand your viewpoint. I'm merely offering up my own.
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"Dawn, the hardest thing in this world is to live in it. Be brave. Live. For me." -- Buffy, "The Gift"
"If I had only thirty seconds to live, this is how I would want to live them: looking into your eyes . . . Always remember: I love you." -- Xena, "Friend in Need"