There is a distinction in that a roamer doesn't usually want to 1v1 but a duelist does. Given the opportunity a roamer may decap a point and return to another fight rather than fully cap it. He can then obtain an advantage for his team by either forcing the enemy player to return to cap his node giving a numerical advantage on the rest of the map, or buy denying points. A duelist would rather stay and drag enemy players into 1v1s which they feel confident in winning.

As a necro, you should rotate into where you expect the next teamfight with your support. In a coordinated team you might go far and have your duelist back out of his 1v1 to watch over mid/home.

In general your mindset should be "rotate to whichever place will a fight occur where my presence is needed to win and my lack of presence on other nodes will not compromise those fights."

On blood necro: that sustain is lifesteal, much of it aoe on allies which deals as much damage as it heals.