Speaking personally, I can't see what reason anyone could have for NOT joining Mudcat. I've been around here for a while, no-one has tried to sell me anything, spam me, or abuse me in any serious way.
If I wanted to post anonymously, I could use my son's computer or delete my cookie (and later reinstall it automatically) as I've done several times whilst chasing viruses/Virii?

I cannot see any advantage in NOT joining, providing you intend no mischief.