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Thanini is made from sesame seeds and therefore has a different flavor then PB, I have made hummus without thanini but I sub it with sesame oil instead, I don't know if I would like peanut butter in hummus or not.

In the middle east it is not thought of as a spread in the same way peanut butter is. it is generally thinned out as the quote stated, with water, lemon juice, cumin, s&p, and sometimes a touch of vinegar...it's thinned out to a pancake batter-ish consistency. At this state it is used as a dip for pita bread and as a dressing on sandwiches like falafel and a hamburger called kofta. (My dad also used to put thinned tahina on a plate with some molasses and eat that with pita...he claims it's something others in Egypt do...but i'm not sure)

Tahina can also be used in a sweet candy called Halawa ( I think it is sold here in the states as Halva) that is a sweetened tahina often with pistachios.

I use 2 tablespoons of PB and a few drops of sesame oil in my hummus when I can't get tahini (because unfortunately my supermarket NEVER carries it). You really don't taste the peanut-y flavor, but if you omit the PB or tahini the hummus just isn't rich enough for me.

Alton Brown's recipe from Good Eats suggests substituting PB if need be. It's a keeper.

I think PB is much sweeter than tahini, and texturally is quite different...tahini, in my experience, was much thicker and not quite so creamy. I've never tried using PB for tahini in hummus, and never even thought to do so, really. I think I'd much rather a less sweet butter in my hummus, like a cashew butter or something, if Tahini wasn't available for some reason.
I used to dip dates in thinned tahini as a snack. I totally overdid it though, and don't think I could stomach them as a result, now. But they were good, in moderation.

I've only ever used tahini to make baba gounoush (eggplant spread) or hummus. I've never tried to substitute PB and can't get my head around using PB for tahini.

I make tahini sauce fairly often (it's great with so many things, from salads to falafel to grilled meats to just pita or pita chips), as well as hummus. I would never have thought of using PB and tahini interchangeably because, well, it wouldn't work for me, these are two different animals. And I would certainly never eat tahini in its raw state - much as I love tahini sauce, pure tahini on its own is rather nasty.

I agree, I would never sub PB for Tahini in a recipe but I've come to prefer it on toast - it has an interesting texture and is much less sweet than even natural, no-sugar-added PB. Tahini and honey on toast is delicious. And veggieout, when I worked at an Israeli company my bosses would use up a jar of tahini every couple of days spreading it on bread and eating tahini sandwiches. So as a spread, sometimes.