This was the response from my ticket with WHMCS Support.

Thanks for coming back to us.

The feature was introduced to give more flexibility and allow for expansion in features in the future. It prevents the need to have multiple services listed under a client when just multiple quantities are needed. However, I can see that the ability for the client themselves, rather than an admin, to be able to upgrade and downgrade the quantity could be extremely useful.

With new feature requests, something that can help is starting a discussion topic WHMCS Community to see if others have the same problem and if so, getting them to add their vote and opinions to your feature request. While we do not implement features based solely on their popularity, requests that get a lot of traction in our feature requests tracker do get reviewed by our team.

If there's anything else I can do for you, please don't hesitate to let me know.

Kind regards,