Serial keys are a form of Rights Management. Some of mine were printed on the Jewel Case and therefore aren't digital at all. If the game never phones home, and doesn't limit my installs or anything, I consider that DRM free. Having to have a serial key that was issued by the manufacturer is fine as long as we're not verifying that against an online registration server. Again. In my and many others' opinions. Not everyone's, obviously.
Considering the publisher/dev can't deny you playing the game via a CD
key (unless they give you a faulty key to begin with
) then it's perhaps the only real acceptable copy protection I'd take. It's non intrusive, works offline, and otherwise gives you no issues. If you lose your key, there's keys enough if you search for them, so that isn't an issue either. (Although NWN is slightly annoying since it won't connect to other games with the same key...
Requiring a key to use the online/MP
component is annoying, but I've yet to actually need/want it.