I’ll add my name to the list of people who would like this feature to be available to all users.
Our users can freely invite people by email address and we see no reason not to allow them to invite via a custom link too.
You could even add a setting that allowed people to set a TL level for this feature, defaulting to TL4.
The problem there is that the person who created the link then gets the credit (badges, etc) for the users who join. Rather than the person doing the actual legwork and inviting people.