Yes, they all work beautifully! There has to be some other great function they could serve for! Wonderful question!

 1) Single speed chain tensioner; put in the cable backwards through from the rear derailleur and cut to desired length, and Robert is your mother's brother!

2) Shoulder mobility rehabilitation pulley; combine lengths of chain to run through the derailleur down to and through a hollow handlebar grip on either side. Hang the derailleur from a hook on the ceiling and bingo!

 I had that 9sp XT for 8 years! My 10sp SRAM X-9 is challenging it. In use since 6 years. Survived 4 derailleur hangers! 10sp Zee/SLX/Deore are a bit crappy though, easy to bend the parallelogram, cage is made of cheese and it’s hard to make it work again. I did save 10sp slx getting it to 90% of its oroginal state, but my Gawd, it went through torture with vice, pliers and a hammer took me an hour or so of realigning it.

 Yes, just cleaning up to make room in my shop. Too much old clutter but they are all packed with tons of great memories.

 @nikoniko: I should do the same some day soon. Make space for something new!