1) I view it as giving back, to some extent. When I first started doing post work, I actually probably learned as much about doing it from editing other people's photos as I did my own. To start, with my own photos, there was this sort of emotional attachment to them, that probably sort of hindered my ability to learn post very well. Working with other people's photos, there wasn't an emotional attachment, and I could really gain a better feel for what my 'post style' was, and how to do things, etc. I think allowing people to edit my photos possibly allows others to learn in that same way and get better with post.
2) Generally if I put something up here for C+C, I'm not totally happy with it 100%, but still think it's good enough to warrant showing and want thoughts. Even if I ultimately disagree with the edits, somebody presenting those alternatives and making me decide why I ultimately liked the way I edited it to begin with gives me a stronger view of my artistic vision. So, either they showed me something good that I didn't think about and helped me improve the photograph, or they showed me something I didn't like as much, but in the process helped me better understand what I like about my art, and in defending it, why I like that aspect of it. Either way, good has come.
3) in cases where somebody does a poor, not well considered edit (which is what many of the 'not ok to edit' people seem to be afraid of), most of the time other commenters will quickly come on and say they prefer the original.
4) You gain valuable insight into how viewers take your work in. By seeing their edits, you get a feel for what they saw, what they saw as needing to be enhanced, changed or deleted. I have an okay idea about how I feel about my work, but it is helpful to visually see how others feel about it as well, to know if Im communicating what I wanted to get across. If someone totally changes my photo to show something totally different, it may mean that I wasn't clear enough in what I was trying to convey for them to be able to 'get it'. The artist cannot complain that an audience doesn't get their work. Part of the job of the artist is to enable his audience, whoever they may be, to understand their work correctly. If the audience doesn't get it, the artist has failed, not the audience.