A sig ban is useless. People are naive
A lot of the banned users are already using another account and they won't come back to use the previous
An account is your identity on bitcointalk. Sig ban is the minimal punishment for them. They can still use the forum, post using their original account. That's the purpose of sig ban.
The above user cellard is an old member, have good quality post too. Cellard has built an identity here since 2014, if he get banned, he can never come back with the same identity. Therefore, sig ban is the best for now.