I wasn't comparing guilds to raids, I was comparing guilds to guilds. Your 5 characters technically aren't a guild, they're half of a guild; but Blizzard is giving us a little leeway by saying 7/10 and 20/25 is acceptable. Those 3 sentences I wrote were their own separate ideas.
Because Blizzard said so. They obviously want > 50% of the raid make up to be from the same guild.
As Khatovar said, how difficult is it to leave an alt in your guild to hold the bank? I have my main in a raiding guild, where we all share the guild bank. I have 4 alts with 4 different guild banks holding all kinds of illegal goods from south of the border. My main will continue to get achievements and guild perks, while the alts, that I could care less about, don't get squat.
TIME TO START 10 BOXING!