Why must the long cross-country flight for a commercial rating be solo? Assuming I am interpreting the below FAR correctly, one must either be solo, or have an instructor present to complete the long cross country requirement. More specifically, it seems to me that you could meet the requirements with a passenger present if an instructor is on board.

§61.129   Aeronautical experience.
(4) Ten hours of solo flight time in a single engine airplane or 10 hours of flight time performing the duties of pilot in command in a single engine airplane with an authorized instructor on board (either of which may be credited towards the flight time requirement under paragraph (a)(2) of this section), on the areas of operation listed under §61.127(b)(1) that include—
(i) One cross-country flight of not less than 300 nautical miles total distance, with landings at a minimum of three points, one of which is a straight-line distance of at least 250 nautical miles from the original departure point.

I'm trying to understand what the purpose of the solo requirement is, and how it could be alternatively achieved without being solo, but only if an instructor is onboard.
It's clearly not for safety of the passenger, since, as a private pilot, its completely legal for me to make the very same flight with a passenger; I just can't use it to meet the commercial requirement quoted above. Why not? And how could adding an instructor to the plane make any difference?
I got this answer to my question from a CFI...

The idea behind the solo requirement is to assure the pilot can accomplish the trip unassisted. Other passengers are permitted if an instructor is present because the instructor will assure that the passengers do not assist with the flight.