Board Thread:Dark Souls II/@comment-25966281-20150311034454

Just to answer this common question in a nutshell, here:

Healing in duels or sparring matches is considered cowardly, plus it also leaves you open to attack.

During an invasion however...

Anything goes, as long as the host can find an opportunity to heal, they can. It gives the invader more challenge, and it makes sense.

An invader can come at anytime, so when the world host gets invaded, they don't expect a thing. Therefore they get to do whatever they need to do. Simple as that. 