It's obvious that a project with 10 people requires some form of Source Control. But I find source control still very valuable for even my 1-person pet projects, for similar reasons as to why it's valuable for larger projects:
1. Source-control lets me make more discrete changes. 2. This makes it easier to track down recent regressions. If I notice something just started breaking, I can do a diff to see my recent changes and thus greatly limit my search space for the bug.3. This also gives me the confidence to make more bold changes. I can see exactly what change I'm making, and in a worst case, I can always revert it. 4. It makes it easier to edit my projects from multiple machines. For example, I may want to do most of my work on my desktop machine at home (when I'm not playing Civ4 on it, of course). But I may also want to do parts of it on my laptop. And I may want to pull up the source from a machine at work see to get a code snippet for something.
So while source control is not an absolute necessity for small 1-person pet projects, it's still a very useful thing to have.