How is my example misleading? The example was about how you do business, not what advantage you would get from doing the work (other than getting paid, of course!).
It must always be up to the client: if they don’t want to let you use the material they bought from you, then you shouldn’t punish them for it. If I was a buyer looking for a service on Fiverr, and I didn’t want to show it in the portfolio (the reasons don’t really matter, since I’m the one paying you to do work) I would promptly look for another freelancer if you tried to charge me extra for not showing the work in your portfolio.
In a competitive marketplace such as Fiverr, it would be easy to find someone else who can do what you do (most likely) and who’s not forcing me to let you publically use the work I paid for.
It’s that simple, really.
With all that said: if you want to force your buyers to pay more to keep you from using the end product, and your buyers are willing to do that, and you’re willing to spend time explaining that to every buyer, then sure, go right ahead.
But I don’t think my example is misleading, because it simply shows you that when you buy a product, you own it, and charging clients more is not something I view as beneficial. But whatever floats your boat, I guess.