There's no set value in that. You just can't pilot an airplane without a license.
As a student pilot in the US you can fly without a passenger but also with other restrictions like your flight instructor has to give you an endorsement to fly and most won't give you an unrestricted endorsement. It is really there to get you a certificate and not as a way to bypass it. All other require a certificate (technically it is not a license but rather a pilot certificate). https://en.wikipedia.org/wiki/Pilot_certification_in_the_Uni...
"For example, the assumption can be made that a person who elects, without pilot qualifications, to operate an uncertificated vehicle alone is fully aware of the risks involved. This assumption does not hold true of a passenger selected randomly from the general public. Persons in the general public will likely assume that the operator has certificated pilot qualifications."
Also, not all airports allow ultralight operations.
https://www.usua.org/Rules/faa103.htm
Also, for insurance and legal reasons, no professional CFI is going to endorse you as a student pilot on an on-going basis.
Agreed about the student pilot not getting an endorsement on an on-going basis. It's too much of a risk to them.
1) Passengers are assumed, correctly, that they cannot evaluate the risks of a flight, and FAA regulations must protect them.
That's why you won't see passenger drone flights in this century in the US, or any advertised "ride sharing" between strangers.
(The FAA has been slow on banning helicopter skid pop-out floats for sightseeing flights, but I predict that will change.)
2) That VFR and IFR flights must be physically separated.
That's why everything that moves will be required to have ADS-B out.
If you're involved in any kind of business model that opposes the above, it is only a matter of time until the FAA stops you. You're welcome to use the words "doctrine" or "never" when explaining the above.