My vote is to constrain growth as much as possible, at least that which comes from stupid sources. Smart hackers will find this site just fine without Yahoo or MSN, probably even Google. As "evil" as blocking sites and crawlers may sound, I think these types of measures will be necessary to preserve the quality of content here. Whatever actions further that objective have my vote.
IE users aren't smart enough to change their defaults; Safari users are mindless Apple fanboys; Opera users are weird and talk with funny accents. Smart hackers will take the trouble to launch FF to access News.YC.
Anything to preserve the quality of content here!