I always hesitate to even use a service that has customer IDs. Most of the frequent flier programs have numeric IDs and there's no way I'm going to remember the

I always hesitate to even use a service that has customer IDs. Most of the frequent flier programs have numeric IDs and there's no way I'm going to remember them. It makes things worse because you don't always have a password manager-enabled device when you travel. There is also a bank in my country that your login username is first name+birth year. It's even worse than an email address as username.