I personally support not wearing your letters when you drink. I don't drink for personal and religious reasons, so I don't support alcohol at all. Either way, GLOs are "famous" for drinking and partying, and that isn't really fair because not all groups/chapters are like that. But let's say a certain fraternity on campus has many brothers who get drunk all the time and do stupid things. People will find this out, and many won't take them seriously. It doens't represent Greek life in a positive light.
Also, many students like myself are opposed to alcohol for various reasons. If the GLOs on their campus are always drinking and are well-known only for that, they wouldn't want to join. On my campus, the fraternities that aren't famous for being drunk all the time are the one that most independants respect the most. Whether you think it's fair or not, it makes sense. Greek life doesn't have a very good image as it is; why make it worse?
My point is, drinking in your letters is like wearing a banner that says, "hey all I do is drink!", whether that's the truth or not. SAIs are permitted to drink in our letters, but I would be highly offended if any of my sisters did.
Yet another two cents from sairose.