I only play them if I get a good set of perfect weapon upgrades for something and want one of those skins. I do wish they could be transferred to PvP characters....
Played it last week twice for 2 weapons. I want to go back to it and explore all the discoveries as I haven't got around to doing that yet and I just want to replay Saul's just for the lulz.
I have to admit that I only finished Gwen and Togo. I sometimes do Gwen again if I need a quick weapon for one of my heroes (I guess Gwen is the fastest mission). I always wanted to finish the other two, but always decided to do something else.