That is also a good point if newbie accounts are allowed then there will be lots of new accounts will be registered here then it will make the forum into garbage shit posting forum.
True but that's not the main point why most of the campaigns aren't accepting any newbies anymore.
The signature space for newbies - jr.members are not applicable for what the project initiators are looking for advertising their websites/projects/services.
It's visible to other members but it's not that really attractive because there's a limitation on the characters, colors and font size.