A key quote, from our then-moderator: "Yes, it's illegal to use a keygrabber. For the reasons I stated in an earlier post. Also one who uses a keygrabber takes the chance of downloading a trojan or keylogger. Don't tell me this is not true because it's happened.
As to gameplay gaining an unfair advantage, the argument on that point is...well pointless in regards to either SP or Battle.net because neither ATMA or a Keygrabber will give one an unfair advantage. However being that using a keygrabber is a voilation if Blizzard's TOA and EULA, it's not allowed.
A keylogger is indeed against the forum rules. If you have trouble keeping track of your cd-keys, then next time register the game with Blizzard at www.battle.net. It'll allow you to download and install it whenever you want without having to use the cds. It'll also provide you with a cd-key for installation purposes.