To beat Twitter at it's own game. Google may not cache twitter live, but it's search is more powerful than Twitter's. A big selling point to Twitter (as in future way to make money) is in its search. If Google can do it just as good or better, why would a data miner hirer Twitter? They will just hirer Google. Why does Google care/possibly hope that Twitter dies? Google Wave.
end of the day you get a back link from twitter.. and google scans it.. so its nothing to complain or worry a
Google crawler from its resources and data center is not that very high, it does crawl but somewhat filtered its result
google bot crawls all and everything on the web that's not username and password blocked or directed by meta tag instructions not to be indexed or crawled