Automation always pays off. Simplifying the process always pays off. Providing yet another step to the workflow would be a move in the opposite direction*. Is there any point in weighing each time whether a mistake is common enough to be included in the commit hooks? It's not like we're paying some SVN vendor a fee per hook ;-)
No, but it's not like the hooks appear by telepathy either. Someone has to write them and to maintain them. I'm not sure who that is currently (Martin is a safe guess ;-)) but, regardless, it's not "free" in terms of maintenance overhead.
I personally don't care whether we deny non-breaking spaces or not. I see no reason to deny them, since the cause of the test_trace failure was ultimately a bug in the trace module, and the non-breaking space actually uncovered this bug (isn't uncovering bugs a good thing?). The interpreter has no problem with utf-8 characters in source files, and I guess most humans have no problems reading non-breaking spaces either.