Running continuously is usually the ideal point. For heat pumps, it definitely is as the efficiency is highest with the lowest split between indoor and outdoor temps.
The issue is that if you suddenly want more heat, you first have to raise the water loop temperature before that can start pushing more heat into the house.
Systems are usually designed to keep up at perhaps 22-24C even on the worst days of winter; maintaining 17C is a lower target that can be met with less capacity and cooler radiators.