One mercs can equip their ability items (Nader, Arty, Kira, Sparks) during cooldown, while the rest of the merc cannot do that. Unless they had the said item equipped beforehand (like Fletcher’s stickies) or it switches to last used weapon, right after using it (like Phoenix’s Healing Pulse or Phantom’s Refractive armor).
Why not allow all mercs to equip their ability item, just to prepare them to be used right after the cooldown ends? Keep in mind, this is coming from someone, who equips item abilities, and doesn’t rely on quick ability key at all.