It's better for everyone if you just let it be untouched and unlimited. Of course, if you see that problems are being caused, it's okay to set a limit through webmaster tools. It won't have a huge effect on anything, really. If you have the opportunity — don't change it. "If it ain't broke, don't fix it."
Well of course you want your website crawled. But leave it be, Search engines should naturally do it.
I think it only needs to be changed to higher if your content changes every 2 minutes. I would never lower it.
I've learned that the setting in Google Webmaster Tools is just a suggestion. Found a way to rapidly speed up crawl rate without having to touch that setting.