How much did Terraria get hurt by releasing non-obfuscated binaries at first, and then having hackers grab the source and release their own version? They are probably the main case I can think of for a C# game that was released without obfuscation and was popular enough that someone cared.
(Though as a counter-point, someone got annoyed with FTL's lack of moddability, and just recoded the whole thing from scratch and used the same art assets)
I'm kind of on the fence, doing a mild amount of work to give hackers a slightly tougher time might be worth the effort, it's hard to say.
Woho, I didn't hear about the Terraria thing. I'm Googling that.
Shouldn't there be a license like Creative Commons Attribution NonCommercial included with the game?
[Added]
Well Terraria dev said "I didn't obfuscate it for a reason". I guess he wanted the community to mod his game, which is cool.