MLS, the role of FTO - Flight Test Observer usually means more preparation than the pilot, often greater technical knowledge, more often than not planning and briefing the sortie, and almost invariably writing the report about it and doing a great deal of detailed analysis.
I've done this for a wide variety of tasks including erect and inverted spinning, transonic dive bombing, short field landing performance, asymmetric handling, civil and military crosswind handling, military helicopter OGE hover, performance of a 1930s biplane, dropping a parachute load from a Hercules over the Irish Sea - I could go on but would probably bore you. This experience is entirely equivalent to flying solo in terms of flying experience, albeit clearly different.
So, I maintain that my 40ish types in other roles is just as valid as my 50ish types as pilot in command including half dozen single seat types.
I have been a passenger, in NR Fairy's R22, numerous airliners, the occasional military fixed or rotary, and hitched a ride in a few light aircraft that happened to be going where I was: those I didn't and wouldn't log.
And it's a pretty poor instructor who allows a student to "ride along" as you put it.
Yes solo flight, particularly in a single seat aeroplane, is a quite unique experience - in that I agree with Cecil Lewis enormously, but it's only a small part of the overall experience of flying and learning about flying.
G