The only way I could see it working, is if the "demo" was based on the same engine, and had the same mechanics, but was actually different to the core game itself.
Kinda like an episodic prequel or a stand-alone expansion, (like the demo, actually adds extra stuff to the game when you have both).
Using a game like Call of Duty as an example to explain, the "demo" as it were is the multiplayer part with unique maps unavailable in the main game.
The demo is significantly cheaper, but it has the limitation of a lot less perks and extra weapons the main game has, as well as having no single player stuff, nor the 10-15 maps the main game has available.
The benefit of combining the two would then be having access to those maps with all the extra stuff the main game has.
It would also be nice, if after a consumer has purchased the "demo", they get a code/voucher (or other redeemable), that they can use to either get a discount on the main game, or the code/voucher gives access to extra dlc.
But knowing the big corporations, its likely that a demo that you pay for will pretty much be a demo that you pay for! XD