I agree with Macguba. I have learned more from decrypting maps and reading .cpp files than anything else. It's the best way to really understand and get ideas on a technical level to put into your own missions. Also I've decrypted and modified plenty of SP maps to play multiplayer with my friends.
I personally don't see any problem with editing other people's work so long as you don't redistribute it. If anyone really had a problem with editing simply because they don't like the idea of them playing their mission the way they want to... well they can take their immaturity and stuff it up their wazoo.
Same goes for addonmakers who make crummy addons that look good and then force people to play with the addon in it's perpetual state of crappiness.
This is just my little opinion. Of course I'm all for maintaining the integrity of people's work in the sense that what they make should always recieve due credit. I do think that the OFP community's greatest asset is that it's an open one.