You know what, I didn't even realise Juniors couldn't have a clickable signature since then. I swear I've still seen Juniors with them. I'm a strong proponent of removing signatures completely from lower ranks and also requiring at least one merit to become a Junior, but I don't think it would effect traffic one bit.
It was implemented right before the Merit system was.
Jr Members can no longer have signature links.
At the time I assumed with the objective of excluding them from signature campaigns but that didn't work as they just use text versions of the URL. I think Google's crawler must still see it as a backlink so it is still worthwhile to advertisers.