I somehow find it better that the people who plan it also implement the whole thing. Otherwise, such matters end up in mutual blame.
No, the fact that the architect neither bricks nor carpenters does not harm the houses.
Otherwise, pure planning is of course worth its weight in gold, as it is planned in the financial interest of the client and not in the financial interest of the planner, seller, and installer.
That's right. Since 1991 (with telephone systems, not houses), my profession has been pure planning, with no selling interest mixed in.
There are - increasingly in the general high-tech trend - more and more things whose complexity requires specialized consultants, as top-notch mechanics often are not good writers or have little ambition to do so. The days when light was simply "on or off" are over. This turns more and more electrical work into electronics. Many electricians are not really fit for this and can only sell something from their wholesaler's catalog, but would still be able to connect it perfectly if it were planned by a specialist.