I definately agree that error reprting should be turned off on a live site. this will help a lot with any issues that setting the file permissions will have.
It's still not the best idea for a plugin. The biggest reason is what happens if the file permission gets set somehow else? I've had times when a server admin has done a 'chmod -R' on my file system to fix something that they thought was wrong, and if someone did something ike this, it would change back the file permissions, so it would enable writing to that file again. There's also the idea that most servers are set up slightly differently, especially on shared hosting servers, so you should never count on what the security and error reporting settings are unless you explicitly set them yourself. It may not be an issue, but if you are distrubuting a plugin then it's not a good idea to have "change file permissions" in the instructions because a whole lot of people will miss/ignore that sort of step as thye won't understand it. If it's installed through the automatic installer then they won't read the instructions anyway and will miss everything like that that you need done.
By setting up important files outside of your plugin structure, you get around all of these issues, and all it takes is one or two lines of code in your plugin to craete the extra folder if it doesn't exist already.
Changing the file permission isn't wrong, and it will work in the vast majority of cases. But it does create it's own set of concerns on top of what you start out with that can easly be removed by using a different strategy.