Many larger organizations need to add Sauce users on a weekly or daily basis. Currently, this responsibility falls to the org admin (assuming that SSO is not enabled).
Allowing team admins to add new users would extend the delegation capabilities of XTM. Since it wouldn't be secure to allow team admins full rights, this capability could be limited. (i.e. can only add to a single team, and only the Team Admin and Team Member roles available).
This could potentially be an option that the org admin could enable/disable in the Organization Settings. This would allow organizations to keep user creation centralized or delegated as they prefer.