An argument should be able to stand by itself. Remember, when discussing something, you're discussing the points, logic and facts. Not someone's experience. Now fine, they can present their experience/observations as points/facts, and if they're true, then they become part of the argument they're making.
*Edit. Fixed bad grammar. My bad.
The nerve!
Seriously though, I think there are two kinds of arguments - logical arguments from validated facts with statistical control, and opinion arguments from some amorphous expertise.
The former can always stand by itself. Ideally, that would be all we ever need, and we get by in hard sciences quite well with just that.
However in softer areas of human knowledge we don't have the luxury of repeatable experiments, so we have to rely on "expert" opinions. I propose we restrict this to the opinions of people with first-hand experience, because if we don't we're running risk of baseless arguments that will never converge. When two people argue from personal experience, there is always an opportunity for them to compare the direct experience of each other and resolve their differences. When people argue from someone else's experience that opportunity is gone, and all that is left is two people sticking to their believes no matter what.
You will note there aren't any facts mentioned in the GGP post (by trhway), only opinions. I think it's ok to do that, but only if it came from direct exposure to the subject matter.
We should probably make it a pre-requisite for anyone who wants his/her opinion to be considered. First-hand experience is invaluable.