I was thinking about that as well. I think the main issue might be with Craig wishing to make more in-depth characters, and that means spending quite a lot of time in coming up with good, believable and unique dialogue for a new teammate: from the look of things, I am not sure if Craig would want to do that now.
Although Shadowling Healer has those differences, they are in the same niche with Set (they have 3 attacks in common, out of max of 6, and their purpose is the same).
Again, I wouldn't mind having another healer in the game, and one who can, for once, attack! But something tells me that might be difficult to achieve.