The only conceivable reason not to become a member would be the desire to remain totally anonymous. There are no hidden costs or promotional emails to worry about.

The only disadvantage to becoming a member is that people sort of get to know who you are after awhile (or at least think they do), and that can lead to repetitive crap and arguments with various people who just don't like your style, and that can get annoying sometimes. But it's a minor problem, I'd say.

There are no financial or procedural or technical risks involved in being a member.

The only thing you will achieve as a result of member status is that you will be clearly identifiable AS you whenever you post something. That's it. Period.

Example: You can always post as "Guest, the idiot"...but then some other Guest may also post as "Guest, the idiot" after awhile when people have gotten to think they know who YOU are, and he may pretend to be you and further say that he hates Jews, blacks, and women and is in favour of child pornography. Then a lot of people will jump to the conclusion that YOU are a racist, anti-semitic, sexist, criminally perverted pig. That can be damned inconvenient, even though nobody knows who "Guest, the idiot" really is...

But, if you become a member and have a member name, such as "Charlie the Tuna", then no Guest can anonymously log on as "Charlie the Tuna"...they can only log on as "Guest, Charlie the Tuna". This will be a signal that they are an impostor. Thus your reputation will remain relatively unsullied when they claim to be you and admit to performing grotesque acts upon young penguins or something like that. That's about all there is to it, really.

Members have more credibility than Guests because they are more identifiable.