If John's rubbish ended up in the merge down to trunk, then you absolutely do want to know about that individual commit.

If you've never received a 500-line patch with a one-line error in it, you haven't been developing collaboratively for very long. Me, I want all 10 commits so I can see from the commit message why the third commit broke the build on platform X, the one clearly tested only on platform Y. I don't want to disentangle the whole mess-o-hackage with a single vague commit comment ("added the foo feature") when faced with such problems.

Am I going to want each and every commit for all history? Of course not, but they're cheap to store, and Fossil makes it easy to ignore the ones not immediately in front of you.