Wouldn't guestimation cause problems (on user's end)? I don't know, but how browsers/curl/wget/crawlers/... react if you tell them Content-Length is 1000 bytes, and then send them 900 bytes and close the connection? or overshot and send 1100? I have a feeling that they wouldn't like it, or at least it varies between fetcher library implementations.
RogerE states above that under-estimating does lead to resource truncation, and over-estimating leads to the browser waiting until timeout/connection break in case there's more data to be fetched.
