they held the extension system back possibly because it could have been a bottleneck or because they just didn't want to give you the possibility to use an extension that blocks their ads. however, they are working on the extension system (http://dev.chromium.org/developers/design-documents/extensio...) and you can already start creating your own extensions (http://dev.chromium.org/developers/design-documents/extensio... or http://dev.chromium.org/developers/design-documents/extensio...).