I like the idea behind the placeholder-system in general. It allows people to go spawn a unique if they want to, but also makes it harder to farm it. However if you are bit unlucky with placeholders you can be killing placeholders the whole weekend. We've run into that kind of lack of luck quite a few times now. Killing Harbing of The One 15 times before The One spawns, killing The Pest's placeholder for 6 hours before The Pest spawns - just to kill it in less than 4 minutes, killing Notum Trainees for 6 hours without soldier spawning only being able to provide every soldier on Rubi-Ka with boosted Hellfury-upgrades meanwhile and so on. Spending 6 hours killing the same mob over and over of which you gain nothing isn't really fun. Most of the placeholders don't give anything (no xp and no loot). I like the way placeholders for Ljotur works, and how you spawn Ian Warr and crew. They give XP and/or loot occasionally.
My suggestion is a change to the probability the real mob spawns. First time you kill a placeholder there is a 20% chance that the real mob spawns next. If another placeholder spawns and gets killed the probability is 30%, and next time 40% and so on. This way you know that you eventually will spawn the mob with a little effort without wasting a whole weekend on it.
How does that sound?