Hello,
That's not the point. If a message is customized by replacing {{SITENAME}} with Wikipedia, updates to *that message* (not to the site name) from TranslateWiki (or changes to the message in MessagesEn.php , in the case of English) will not get through because the local customization containing the old version overrides it.
I understand that. That is the point, once it is overridden, it doesn't reintroduce faulty behaviors.
That is so sad - considering the amount of updates that go to those core messages constantly... ;-) I think both tagline and pagetitle and others will suffer greatly from this. And 'About' message really needs constant updating too :) Seriously, folks, you're enjoying the theory of localization so much, that you forget that there's actual practice out there.
Domas