Is this worn in addition to a gorget and/or aventail? Looking at Sir Ian's avatar, I don't see how you'd ever notice the pisan under the helm and aventail.
Sir William, the pisan is worn in addition to an aventail and functions as a gorget. Rigid neck defense wouldn't really be a thing until Agincourt era as it was attached to Great Bascinets in place of an aventail. As a stand-alone piece of rigid armor for the throat it wouldn't really be a thing until much later. The true 'gorget' that must people think of when they hear the word is a 16th century invention. There only 'gorget' style armor worn before 1415 is just maille, and then through most of the 15th it was the bevor with a sallet, or the wrapper on an armet, or just a maille standard.
it seems a bit redundant if you have an aventail with a padded liner (that goes out beyond the helm itself). I can see how it would help protect against an attack where a simple aventail wouldnt work.
Basically if you have the right type of liner and sewed the end of the aventail to the liner like in the pic above, would you ever use a pisan?
Belemrys, my bascinet's aventail has a sewed in padded liner just like the picture you linked. Yes, you would definitely still want a pisan. The aventail does not protect from thrusts that originate underneath the level of the aventail hem. The pisan can be worn under the top of the breastplate. So you can imagine that if an errant blade were to skip off the top of the breastplate and underneath the aventail, the pisan is still there to hopefully prevent penetration of the throat. The top of the breastplate acts as a means to pin it down, ensuring that nothing can get to your throat without going through it first.