It is not mandatory to place a smart thermostatic valve on every radiator. We however do recommend it, because if a radiator is not fitted with one it cannot be controlled.
Indeed, if a radiator is open without a smart button, it is always heating when a zone requires heat, even if you do not want it warm. If it is switched on, but all your zones are up to temperature, it will never get hot. Not even if you want it warm. When it is closed, it never heats.
Keep in mind that you need a bypass in your system when you switch to smart valves. If you want one of your radiators to serve as a bypass, it must always be open.