That is, obviously, one possible way to frustrate a deposition.

However, with a few exceptions (e.g. privilege, protective order, abusing the witness), they do have to answer the questions I ask in a deposition. I can ask immediate follow-up questions, or impeach them if they give contradictory responses. If I think they're refusing to comply, I can go to the judge and get an order to compel or various other things.

The deponent is the only one allowed to answer questions; their lawyer has to sit there and shut up, except for very briefly stating objections. Coaching the witness isn't allowed.

If a regular deponent gives answers like "I don't recall", I can ask about their memory, ask about other things before or after to see whether they have selective amnesia, etc. If a 30(b)(6) deponent gives answers like "I don't recall", then I get to use that against the agency; there's no protection of human memory fallibility.

So it really depends on what exactly you mean by "non-answers".