I'd vouch for implementing a simple encryption, but with a warning for the users that it's main purpose is to prevent customers from just unzipping the files and was implemented for possible legal issues with bought assets.
I know it won't stop a bit more technical person to steal the assets, but I do think that if extra steps are necessary to get the assets instead of just unzipping a file, at least 75% of the people who would have stolen those assets won't do it. I think it's similar when a store leaves it's door for the night closed or locked.
I agree with what glerikud said.
XOR encryption surely won't stop the big guys but it would be much better to have XOR in place,
instead of leaving the majority of our game assets open for grabs in my opinion.