There are indeed performance issues posed by the initial solution, but from the user point of view it's strange to 'loose' the extension rating after you imstall it. Moreover, uninstalling the extension wouldn't bring back the rating because the extension becomes local. Even for core extensions I think it is nice to display the rating because some are just bundled by default in the war but they could as well be installed (the fact that they are core extensions is a packaging decision, i.e. we decide to bundle some jar in XE but another XWiki distribution could come without it).