While I am in a similar place as @MakoSipper, end game and the trouble of entering codes is not worth the reward, I have only one thing to add.
In a game that is unabashedly RNG based, these codes have always felt like the devs way to "make it rain" and give us, the players, a little incentive (read carrot). If that were the case, why not just make the codes 1 per account, rather than have a usage limit. Nothing they offer is earth shattering or game-breaking, just allow anybody to benefit from it if you plan to offer them, imho.
I know that doesn't solve the issue of one person entering for the entire guild... BUT... if somebody in the guild takes it upon themselves to be the CODEMASTER for the guild. I don't have a problem with that. That just adds to the comraderie of being in a Guild, and if the codes were limited to 1 per account then it wouldnt matter because everybody would still be entitled to their carrot, em, incentive, regardless to the speed with which those Codemasters input for their guild.
tl;dr- Allow every code to be used once per account and it doesn't matter if somebody is redeeming for their guildmates because it wont invalidate the code for other players.