The advantage of adopting technology and following boiler schematics provided by the manufacturer ...
...
Not following a schematic seems to be a compromise to me... Others may think differently though
Obviously the boiler manufacturers want people to use
their controls - that's why they aren't providing/selling open standards. I see a number of issues with that :
1) It "trains" plumbers to think that proprietary controls are the only way to do things - good for the manufacturer, not necessarily for the customer/user. If all you can do is follow the wiring diagram in the boiler manual then you are not competent (IMO) to be specifying controls.
2) It limits the controls to only what the manufacturer provides - not good for the customer/user.
3) Because of 2, the end user may well be stuck with something that isn't optimal for their situation. It may be as little as the customer not liking the styling of the room stat/controller, it may be that they'd prefer something that has different options but they can't do that can they ?
Conversely, something 'open' (such as OpenTherm, but not necessarily that particular one) means the customer can choose their own controller to suit their needs - without it being influenced by or influencing their choice of boiler.
Just to give some ideas, a had a quick look at OpenTherm and found that several people have done projects to link it into other stuff. One had built a unit which did nothing more than sit in the middle passively sniffing the traffic to log what the boiler was doing. Another built a device that could sit in the middle and take over under control from their computer - as well as monitoring. Both of these would probably be possible with proprietary protocols - though a lot harder since I doubt the manufacturers like giving away any of their secrets.
4) Also because of 2, you may well have problems when trying to do something beyond "combi + rads" or "DHW cylinder + rads" - which seems to be the limit of the majority of plumbers (sorry if that sounds condescending, I speak as I find). How many proprietary systems cope with buffer tank plus second pump for the rads ? How many cope without any problems having other heat sources (solar thermal, heat pump, wood burner, etc) into a buffer tank ?
5) If each manufacturer does their own thing, then installers and maintainers need to learn as many systems as there are manufacturers. If everyone used open standards, you'd only need to learn the open standard(s) plus perhaps a little extra for each manufacturer.
5b) and that follows into diagnostics kit. If everything was (for example) OpenTherm, then you would only need one diagnostics unit to be able to work on any system. But then, I guess the majority of plumbers don't carry any diagnostics kit beyond a ready supply of parts to swap
