In general, it's not a good idea to hack the core code unless you have a good reason to do so.
I agree. A nice thing might be to "hijack" (overload?) that function by re-writing it in the LocalSettings.php - not sure whether/how that'd be possible though...
Are you trying to limit the namespaces for your personal use, or for everybody?
For everybody; I think it's a bit odd when changes on user pages appear on the global RecentChanges list...
If you're trying to keep everyone from seeing changes in other namespaces, the "proper" place to hack the code would be later on in that method - during the SQL query generation step. My suggestion was just to alter the defaults - not limit functionality.
I'll look into it - though it's not really a crucial issue, but rather a minor annoyance.
Thanks for you assistance!
-- F.