There are limitations already built in, but they're easy to miss when thinking in terms of more familiar games.
First, it's only easy to trigger at all, let alone over and over again, in situations that tend to already have fairly low stakes. (Not always, but often.) You have to speak frankly with someone, not at or to them, so they have to be at least minimally receptive to listening to your frankness, and that tends to be when things are already mostly going OK. Guards hauling you away to prison and dragons about to eat you for lunch are either not going to talk with you at all, or are not going to speak frankly with you, except under, well, exceptional circumstances.
But the bartender with a secret, or the duchess with a political plan, who doesn't realise the Bard is a social force of nature — that NPC is easy to get into such conversations. All it takes is not being on guard in the first place, or letting their guard down for even one conversation. Yes, this will sometimes catapult the game's events forward at a rapid pace, as secrets and sordid scandals are unearthed with ease, but speed is normal in Dungeon World and the GM's job is not to keep secrets safe, it's to fill their lives with adventure. Throwing them headlong into the middle of sociopolitical tangles is definitely living up to that job. The Bard's move is more helpful to the GM than not!
Even when stakes are high, the move is still more useful to the GM than you might think. Sometimes it will let you pull the group into an intrigue effortlessly. Sometimes it will let you give them information that will heighten an already tense situation. Sometimes it will let you find out much more about the Bard than the Bard bargained for. In fact, it's a very dangerous move for the Bard to make carelessly…
GM: “… Okay, so now that the villain has explained her plans, she asks in the spirit of frank, open discussion you've established, ‘What could I do to make you betray your companions, right here and now?’”
GM: “And remember, you have to answer truthfully.”
(And “Nothing!” is almost never a truthful answer, while “Uh, give up your evil scheme?” isn't answering at all yet because it would help rather than betray the Bard's companions.1)
The limitations on moves in Dungeon World are always — always, always — the risks that are the potential consequence of making the move, which naturally balance the potential rewards, not difficulty or up-front limitations as in most RPGs. And with this move you don't need a player to roll a miss to reveal the unwelcome truth that their friendliness invites risk when they're being very frank with dangerous NPCs.
1. A very clever Bard might take advantage of such a question by truthfully answering something that the villain would (probably?) refuse to do, or which would further a greater good even while it betrayed the rest of the party. Such cleverness is awesome and is the stuff of epic finales.