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.
In the past there was signature only and avatar only campaigns and others using both signatures and avatars. At that time junior and members have more options to join campaigns.
Few years ago, maybe with the merit system implementation and stricter policies regarding spam, in addition to the availability of relatively important number of higher ranks, it becomes less attractive to recruit less than Full Member.
I would say that the situation of Full Member of nowadays was the same as junior member years ago regarding signature campaigns.
Didn't know there's an update in the signature design restrictions. The last I remember, they allowed junior members to put link and text with a maximum of 160 characters, and 4000 to 8000 characters for legendary and hero members. Could you please provide me with the link to the update?