>>5
A bump alters the structure of the frontpage, lets the still alive threads fall off the end of the page and may hide new threads.
Here threads never expire/get deleted, though, so if it falls of the end, you can catch it from the thread list/second page and bump it if you have to say something relevant/contribute.
If nobody bumps everything and sages his posts, it's less likely that active threads will end in second page thus no the threads will not be filled with "bump" posts to keep it alive.
That doesn't mean that we don't abuse sage, we do.