But 315 is not a magical number, nor does it always point to best dps. People that are lazy are going to skim through forums, see 315 everywhere (since it is everywhere) and be misinformed.
Spreadsheet doesnt work for everyone and some people dont download it sure - but it does not mean to just get 315 anyway and think it'll up your dps. With some gear setups sometimes other gems will outweigh hit gems below 315 and equaling 315, and group make-up will vary that even more.