Having not read the thread: Total first impression.
1. This seems like a really good idea as a mechanic, because it gracefully solves the problem of purification growing tedious, and it doesn't give you a very solid advantage really, aside from just not making you take as many risks to yourself. That being said...
2. The problem I have with currency is that it does nothing as a power-up. Sure, you can hire mercenaries to do things that you can already do, and they reward you by allowing you to skip out on a bit of a repetitive part of the game. The thing is, you can already purify stuff on your own. I'm picking up the coins to be rewarded with less gameplay in the worst case scenario. Best case scenario, coins are nothing but 'fun to pick up'. It takes a lot of talent and effort to make a coin fun to pick up. Games like Mario have achieved this to a great deal of mastery, but there are many other games that don't succeed at it. It's not a core necessity to make coins fun to grab, but if the coins don't add any fun to me (as I doubt I'll hire purifying services) and aren't fun to collect, then I'm losing a game mechanic. As it stands, I doubt I'll hire a mercenary, and the coins are just not fun to collect. I kept getting them because they existed, but not because I liked the jingly sound they created or anything. Look to Link to the Past or Xotic or other similar games for rewards that are really just fun to earn for various reasons.
3. The benefits of equipment are really really cool. This is really the biggest reason, I think. The last one was a whole load of game design mumbo jumbo, but ultimately, I want items that do cool stuff. When you said "power-up", a lightning bolt struck. Not literally, but in my mind. Equipment is cool because it's basically this game's functional power-ups. If you replaced equipment with power-ups that provide a temporary (probably timed) benefit that works the same way as the present equipment works, we don't lose the effects. Ultimately... sometimes, I want to collect triple shot and just go have fun with triple shot. If all of that was taken out of the game, the surface areas become a slog of 'shoot stuff tactically'. It's not as if that's bad, but power-ups add a lot to the game when implemented well, and throw in a bit of variety, a bit of the same twist you get from the cape or fire flower in a Mario game. No, those items don't make the game, but when those are in the game, you have something to go for. The entire game isn't just "Jump and run". It's about the power-ups you can keep, and how you use them as well. It just gives it the 'more' that breaks up the action just the smallest bit.
Now, the problem lies in ganking loot or making the game too much of what it isn't, right? Well, is this not more a platformer than an RPG? Don't even give the pretense of 'equipment'. There's a power-up. Some energy ball. Someone gets it, it goes to everybody in the chunk for a time, and a survivor explains "This is a deposit of pure consciousness energy that provides various effects" or something.
In Summary: It's a graceful little simple solution to make mercenary coins actually be useful. However, I don't like removing the equipment's effects because I don't want to lose out on the power-up effect of equipment, and I know I am unlikely to bribe my way out of the simple pleasure of purification. So, replace equipment with timed power-ups and give merc coins this function so that they actually have a use.
Edit: I really really wanna emphasize one thing, and that's the fact that I don't think the merc coin idea is bad. I just know that I won't use it. The way I look at it, I'm skipping gameplay, and I like gameplay. That's why I'd never opt to auto-complete a level I'm stuck on or anything like that. That's why I'm not a fan of idle farming methods in offline games. I don't like those, because somewhere, somehow, a game is being played, and I'm not the one doing it.
