What is the philosophy behind a customer/user not having a name and only use one in the billing/shipping address? For example, Bob James could order a piano for his friend (who would be the named addressee)

People often times also ask other people to order for themselves. Payment gateways will require exact match of billing records to have payment go through. So if say even a same household person is ordering using a shared card but the card is in the name of 1 person, they must use the correct billing info. Granted you can remove such restrictions at the payment gateway to make it more loose but that will give you a nice quick way to burn cash from chargebacks

I always assumed it was because, from a personal perspective (as opposed to a business), the billing name / address is the same as the customer name / address. If I buy it for somebody else, the shipping name and address would be for somebody else, but the billing would always apply to me.

I agree with all the above, but I also think it would've been nice to have a name attached to a customer account. Just grabbing their default address could be the wrong assumption

I think the idea for Solidus is that it's a barebones framework. There's a lot that it doesn't do on its own, but it's been built with reasonable philosophies. If you need a name on the account, you can add one. That's something we did on one of our sites. We broke out the name into a first and last name instead of being a single name field, too.

Exactly, if you need a name on the user model, you can just add it as you prefer. It is not needed to complete an order most of the times, so it's not there

Fair