Sadly, to some people it's all they have to cling to in an argument. It's one of the most childish things you'll see at FR. It's equivalent to "I have more toys than you do". When I changed my screen name recently because I hated the old one, I was so amused by the constant use of the "I'm bigger than you are" taunt anytime someone disagreed with me. It weeds out the intelligent adults from the tantrum having losers.
On the other hand, there is a comfort level with posters that have contributed thousands of posts since their signup date. You know where they're coming from and you're far less likely to misunderstand the point they're trying to make.
New screen names tend to have a higher rate of troll, infiltrator, and troublemaker than folks with a track history of at least a couple of active years. That's just a fact.
I don't know what the magic number here is. It's at least a couple of months of active posting on a variety of topics. You can't get acquainted with everyone all at once, although some have tried.
I don't know who you were in your past FR life, but if you've been here for any appreciable time you'll know that newcomers are inherently distrusted because they often deserve it.