We currently use aMember. Although it's not super common - we do get a few customers each month that, for whatever reason, purchase a new product using a different email (username), so the system creates another account for them.
This isn't a great experience for the customer as they end up with different logins for different products. aMember has a handy feature that "merges" two accounts together - this way the customer gets one login to access everything.
I was amazed that MM doesn't have this feature.
I second this. In our case today we have a customer who has 4 years of data in our system, he bought a subscription to a second product but used a different email address and now we have an irreconcilable mess.