The best example I can give you of the in / out situation was a Singaporean I met in Paris. He flew out to LHR, back from CDG. I can only assume the price of his ticket was much less than a simple return to/ from either LHR or CDG.
The assumption made for not fare splitting is that the airlines will price gouge for providing a direct service. This is often, but by no means always the case.
In my personal experience, fare splitting just to save money is probably a false economy - but if you actually want to stop at the transfer points nd can take advantage of doing so, then you can extract a great deal more value from your trip. Drag partner and two screaming kids in tow and you can forget it!