Oddly enough, finding out why someone would want to upgrade to 5.8.9 rather than 5.10 is exactly what this project is all about. ;) Without the release notes, I can't tell you for certain. However some things which immediately spring to mind:

• Some regexps run significantly faster in 5.8 than in 5.10. If you're processing massive amounts of data, you may have benchmarked your alternatives and discovered that moving to a leaner 5.8 is better than moving to 5.10.
• 5.8.9 is binary compatible with the previous 5.8 releases. You shouldn't need to recompile any of your modules. Yet 5.8.9 incorporates many of the performance enhancements and fixes that went into 5.10.
• You have an organisation that doesn't want to move to a .0 release of anything. I can think of plenty of these.

Any of the above AND...

• You re-bless objects frequently. This is faster in 5.8.9, especially if they change their overloading behaviour.
• You want to save your debugger history.
• You use the diagnostics pragma, and don't want it to coat your regexps in concrete and drop them off a nearby pier.

There'll be other reasons when we find them. Plus, as chromatic noted, 5.10.0 isn't marked as "fully stable", although I'm not sure what that means either. It's certainly stable enough for me, although I know of a few things that will be changing for the 5.10.1 release.