I did not get that effect. I had to reboot before the two partitions I hid disappeared.The partitions disappeared from places immediately, without rebooting.
But it does work, and I am happy to not see the useless partitions anymore. Thanks for the pointer.
One interesting thing about the bookmarks side effect in Places is that, once there are enough of them, they go into a submenu of the Places menu. That's OK by me.