I think about short urls and twitter this way: Twitter is supposed to be real time so in reality the url only needs to last say a few weeks to really be viable

I think about short urls and twitter this way: Twitter is supposed to be real time so in reality the url only needs to last say a few weeks to really be viable even for search so if there is a pollution of some dead links over time, who cares its not like it is anything new and you should be able to google it by then.