Actually, I have, to determine whether it was gummed or ungummed wallpaper. Besides, if you've ever been to a Hawaiian luau, and eaten poi, that's pretty much wallpaper paste right there.
There's a large number of past collectors that do the same. Before I retired Ivwould watch customers bring on letters and a notebook filled with old stamps. Using tweezers they would remove the stamps from the notebook, like and stick them before dropping in the mailbox. Yes, for you youngsters before peel and stick stamps, all stamps had glue on the back that required it be licked before placing on the envelope. It was fun to watch people lick the new peel and stick stamps when they were first introduced.
I know people that use the end rolls of wallpaper as gift wrapping paper. Less expensive and stronger.
Both the Chicago Coin Club and the Harrisburg Coin Club use multiple old stamps to send out their newsletters, and YES, they do still send out snailmail newsletters, as does Red Rose Coin Club. Red Rose sends out too many to have enough old stamps around. Harrisburg is beginning a monthly TV program on local access cable.