"is the option of having different vehicle classes at disposal in a single world more/less important than the highest possible fidelity of a single simulator type?"
Personally, I would have to say less, but I don't see how that is a choice we would have to make.
Outerra provides visuals, IMO it should do very little else as an engine. Physics model, input handling, etc should come via plugins. This way, eg. Aerosoft can bring a weather engine and flightsim tailored physics engine to the table, along with a base library of say airports. Then another company for instance could plug into that, and provide a visual Airliner, or they could even develop their very own physics model, outside of the base flightsim model someone offers. Someone completely different could do ATC simulation. This way, even more potential depth is achieved.
Stepping outside of flightsim, lets look at a racing sim on top of outerra.
Such a racing plugin would probably bring in proper physics model, as well as tons of carefully designed tracks. (when im in my flightsim helo I can fly over those for instance). A racing sime could also bring police AI, and more immersive city scapes, perhaps pedastrians. As long as lod levellin is done propertly, a flightsimmer can profit from all of those things as well, and vice versa. Race your Porsche to the airport and see AI generated traffic take off over you, something like that.
So, for me personally, I want maximum immersion in a genre. However, I believe the multi vehicle/simulation aspect helps rather than hurts/limits this aspect. I also think that with one united base engine, the modding community will be huge.
Edit: I appologize, removed reference to specific companies.