It shouldn't be a problem

That's not really how Blizzard feels about things like this. When you get down to it, "botting" involves having a program or script or whatever interacting with the game for you. Having a bot actually play the game is obviously an egregious example, but having a program automatically build a deck for you is still bypassing the normal way of interacting with the game.

People have been banned for botting in WoW for things as simple as a bot that fishes (which involves using a "cast" ability then clicking on a bobber) and even just one that periodically clicks or moves the character a tiny bit so it doesn't go AFK. Blizzard has shown in the past that they're very serious about all levels of botting, big or small.

While there's no specific set of rules right now about what third-party pieces of software are legit, the informal rule that's been given repeatedly is that anything that replicates what could be done with a pen and paper is fine. A pen and paper could let you keep track of which cards you'd drawn in a game, but it certainly can't execute the series of clicks necessary to build a deck.

And as far as offering no in-game benefit, that's also debatable. The only thing to be gained from using a WoW fishing bot was to save myself a bunch of tedious time clicking. Using an automated deck builder in HS accomplishes the same thing...I could deconstruct and rebuild decks manually, so all that program does is save me some time. But Blizzard's past stances on botting and automating processes give a strong indication that they consider automation for the purpose of saving time to be a violation.