Patches are as easily cracked as anything else, though.
Well, cracking that "type" of DRM requires re-releasing the entire game as the newly patched package. Difficult? No. More time-consuming than releasing a "nocd" executable that will forever work with that version of the game? Slightly yes.
It's really just a matter of a few extra straws on the camel's back. The lower a game's popularity, the lesser the chance that someone will release a certain version of that game. An executable, however, can simply be uploading to one of 982749874 sites dedicated to that sort of thing, and be left there for the world to download at their leisure.
Let's say I have Civ 5. I download an executable for version 1.0. I'm all set. Let's say the game gets patched. I download the executable for version 1.1. I'm once again ready to rock. No hassles, no frustration (unless the executable hasn't been released in a timely fashion - unlikely considering the popularity of the game).
Let's say that the DRM method requires an "account" using a valid serial to download patches. Let's also say that said patches make major changes and additions to the game. At release, I have to get the base game, with version 1.0. No problem. One patch later, I would have to download a cracked version of 1.1. Meaning the ENTIRE game, all over again. Then you have various virus issues, corrupted files, etc.
Sure, someone -could- release a custom archive with the 1.1 patch ready to install "serial" free, but it's unlikely.
Other possibilities include things like Tages, FADE, etc. These are annoying, but ultimately futile. Someone will just nuke those protections the same as a CD check, it just takes more time and annoyance while the kinks in the bypass are being worked on.
I would say that the ultimate solution is to require a login EVERY time the game is launched, but this didn't work for several games over the past year, and it wouldn't work here. As long as DRM is based on code, someone will take apart that code, remove the offending obstacles, and re-release a defenseless piece of software.