Hello.
I think it would be a useful feature.
Educate your customer and create configuration guide for PPPoE client configuration to avoid bad username/password users.
Great idea. But it from fantastic area. 
Rate limit for authentication attempts in time window per MAC address is a great variant, I think.
Regards, Sergey.
[UPD] similar requests:
http://forum.mikrotik.com/t/limit-radius-authentication-tries/16053/1
http://forum.mikrotik.com/t/pppoe-authentication-limit/39087/1