That is exactly what "modal" means, so, yes, you do want to make a modal dialog.
you said "modal dialog", only $input is modal, the dialog is far from being modal
If you think there's a problem with the wording in the help file or if you think the ,u, behavior is bugged in general or regarding dialog window, feel free to make a post about it.
For my part, I use the u switch because otherwise, the $input isn't tied to my dialog and it makes that $input appearing behind the dialog ie: clearly not what I want.
It is working as I expect it to, except for the problem with the script editor, so I'm not going to stop using it because you are supposing things.
If what you say is true, the actual implementation shows it's possible to do what I want, so I would just suggest it.