Totally agree. PvP guilds are needed in WvW to be effective. For those that disagree have no idea what they are talking about. You join the same voice chat program, discuss tactics on your guild website, use your guild tab to know what is required of you for that nights WvW.
Pugs get destroyed 9/10 against guilds with less numbers. I see it time and time again. If you want coordination join a guild, if you want to join the mindless zerg don't worry about a guild.
IF you have no interest in WvW then you can get away with not being in a guild.