What can be so important in a house that can not be off for a few minutes? If using inverter as backup, nothing stays on when main grid goes down.
Every disconnect I have encountered have a center position where both supplies are disconnected. Must be a reason.
But these manual switches are recommended.
https://www.bluesea.com/products/category/7/38/Switches/Rotary_Switches
It is a matter of convenience, loss of data or programming.
Some clocks reset. A desktop PC would shut down if AC isn't switched quickly.
We would like memory and real-time clock to stay up at least seconds with capacitors inside storing power. Some do.
Using a laptop, it's battery rides through, and external monitor power-cycles.
Ideally telecom gear, which may be carrying your Voip call, stays up.
I think a 12V battery with multiple fused cords is a good way to provide UPS for a number of the communications items.
My battery inverters disconnect from grid with a relay and allow a brief dropout before inverting. If they were inverting (support additional local consumption beyond programmed maximum grid current), they shut off for a moment. Time to restore power is not guaranteed especially in the event of a sag rather than sudden disconnect of grid. The only device I have which gets stumbled is a Voip dongle. I have to manually powercycle it to recover. The 12V battery backup is my planned solution, and will keep router & DSL modem running as well.
Center off on AC switches helps prevent arc across. Of course we need break-before-make, but also want to ensure any arc is interrupted before other source is connected.