Mark Clements wrote:
He means that if you have article [[Wikipedia:Foo]] which links to [[Bar]] and you move it to [[Help:Foo]] then the link to [[Wikipedia:Bar]] will now point to [[Help:Bar]] which is not the originally intended destination. Whilst this may turn it into a redlink, a more dangerous result is if [[Help:Bar]] already exists so the destination changes and nobody spots it.
I'm not really convinced this would be a serious problem, though. For the kinds of applications this feature would most likely be used (such as the "micro-wikis" this thread started from), I wouldn't expect cross-namespace moves to be common at all, and those who did them would presumably know what to expect. Compare the situation with the current transwiki process, where the same problem already occurs and is handled routinely.
Of course, if someone were to feel like implementing it, a "fix link targets" option for cross-namespace moves would be a nice cherry on top of this feature. Could be useful for transwikiing as well, and it might even be possible to make it fix relative subpage links too.