Let's turn the argument on its head. Let's say you added some more rads to the system which added up to 2kW. Would you say "That's only a couple of kW, there's no need to balance those rads"?
If you have a 12kW boiler, 10kW of rads and a cylinder with a coil rated at 12kW the cylinder, when both CH and HW are on the cylinder will grab as much of the heat from the boiler as it can, to the detriment of the CH circuit. The only way you can prevent this is to restrict the flow through the cylinder.
As you point out, this problem does not arise if you have HW priority. The only problem with this is that the time taken to heat cylinders was so long that the house temperature dropped. Better insulated houses and fast recovery cylinder are a help here.
Systems with weather compensation for the CH frequently use HW priority, the boiler temperature automatically rising to maximum when HW is being heated.
