Most merchants and service providers keep your shadow account after purchase anyway. If you are lawyery kind of person you can request deleting the shadow account by contacting them and giving away even more of your personal details.
To be clear for online purchases I think it's a nice convenience to be able to buy without requiring an account, but for flights it's an entirely different game, you often need to provide a lot more PII.
The point is that I'm not forced to create and manage an account and I can do a second purchase without having to remember or reset a password to an account that was forced upon me.