This, a thousand times this. Because I don't want to have problems if/when your company goes under in a couple of years.
Company A sells door/window contact stuff. Company B sells indoor presence detectors. Company C sells plugs to dim/switch/measure appliances.
If they make their communication stack transparent anyone can jump in and combine these products to improve comfort (e.g. appropriately dimmed lights as you walk through the house), reduce the energy bill ("you're running the airco with the window open" or kill lights in unoccupied rooms) or add a level of security ("no presence inside, no appliances on but the window just opened...").
Without it you're limited to the island solutions provided by the service.