With the help of campaign managers, bitcointalk will be more active if they bring back the old way of accepting participants, like accepting even jr. member ranks. Most campaigns now only accept Hero to Legendary Members. I think Jr. Members rank is okay with the payment even if it is only $10 per week or less. This would also help the campaign and save some funds. But assume high amount of spam post once this once is implemented.
The biggest reason is, it's not very appealing for advertising teams because of the extremely limited signature styling of Jr. members -- limited characters and no links allowed AFAIK.
High amount of spam is also possible as you say which could mean campaign teams may have to spend more effort on jr.members compared to others. At that point, I don't think it would be worth it considering the limited exposure their signature would give.