Dilbert is not being fair with his coworker because he is throwing the work already done to the trash, and that is arrogant, if you tell why i didn't do something in one way instead the other after i finish(or one year after), that's arrogant, you need to tell it before (IMO).
To those who think Dilbert is being arrogant... Which is more arrogant? To give somebody a good idea or to reject a good idea that somebody else had just because you think it would make you look bad? You don't know how many meetings I've been in where a lead engineer has rejected a superior idea just because it wasn't their own. That's when I start marking off the days until the project dies a horrible and painful death.
I agree with SoundMan. If no one ever shared ideas with coworkers many of the convenient, yet ingenious products would not be in existent today. Its not arrogant to give a good idea, if he demanded that he follow because he's that much smarter, now that would be arrogant.