You can see their ability if you find a battle against them, or literally here on hearthpwn, it's not a big deal. Why would you not craft a merc if you have the coins anyway ? Even if you're not interested in it, you need it for the achievements and you can't do anything else with the coins.
There are more important things to fix like removing maxed out merc coins from packs and rewards.
It's more regarding if it's worth grinding emperor for Diablo at a rate of, what, ~15 coins x5 chance / 10ish options? So about an average of 7 Diablo coins per if all options have equal chance. Do you invest the 80ish runs into that, or grind for another? No clue if it's worth it from only in-game info because the game doesn't tell you! That is an utter failure of UX design.
Maxed out mercs still giving coins is a different, also bad facet of the system.