But why?! You simply can stop using that merc at dungeon fights! You are able to have up to 11 mercs! So you can have some as reserves!!!
This way you are somewhat forced to have more mercs so you need more equipment... so bigger need on market etc...
The market is already short on goods as is, to be honest. It's been a month that I'm looking for an epic Fighter's Pride, it's not even a rare recipe. Let's not talk about rarer items, such as Anaconda's Prey (I only know it exists because I saw it equiped on someone ONCE).
As I see it, it's important for a game to have as many useful options as possible, even if some people would use them and some others wouldn't.
You know how difficult it is to find mercenaries worth your money, and when you finally find one, it's a bit frustrating to be practically useless without blocking your own XP.