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.
There is a good reason not to craft a merc. You can hope to open it later in packs and save the 300/500 coins that you would spend on crafting, which you can now use to upgrade its powers