For a forum such as bitcointalk it is better to provide local captcha service.
As a result of attacks from 5000+ IPs in 2017, the capcha login requirement implemented to avoid such same attacks.
Furthermore, from want you see
If it causes problems, I might restrict it to Jr Members and above or something, but currently anyone can do it.
Admin has still allowed all member ranks to use capcha code. When you can still be allowed to use it, why local capcha services are needed.
But if someone try to use capcha code to attack, theymos will probably restrict it to higher ranks. I don't think it will happen (attacks) because the forum has become cleaner when shitposters eradicated over recent months.
Reference:
Recently someone has taken to using 5000+ IPs to bypass rate-limits and try many passwords. Therefore, it is now required to solve a captcha when logging in.