Right now, WP Super Cache doesn't let you change your settings unless wp-content is writable by the web server. However, the only files it needs to change are wp-content/wp-cache-config.php and the contents of wp-content/cache.
The plugin should allow you to change settings based on those permissions, rather than requiring all of wp-content to be writable (which, IMO, is a security risk).
Yup, I'm mystified by that, too - all the more given the IMHO very much deserved increase in attention given to and concern over tightening WP security to avert hacking.
I have no idea whether it's a simple thing to do, but it'd surely make me feel more relieved to see that 777 or 757 permissions lifted from /wp-content...
Thanks for reminding me of this. The wp-content requirement is from when wp-cache-config.php was created but I've fixed this in trunk. Hopefully have a new release next week.