This is sloppy legal analysis. If the court was even remotely consistent then the vast number of times that I have had to deal with that answer (and, the followup objection by defense counsel: asked and answered) to subjects that the witness does not want to discuss in deposition would disappear in a puff of legal logic.
On occasion I've let the weasel slide and during the body of the deposition I've inserted questions along the line of:
Are your parents still living? When did your father pass? When did your mother pass?
What was the address that you lived at when you left for college?
Please state all of your past employers that paid you enough to require that you file a tax return?
What is your wedding anniversary?
What is the day and month of your spouse's birthday? (each of the kids follow)
Who was your favorite college prof? What class or classes did you take? Do you remember your grade(s)?
When did you receive the notice of this deposition?
What did you have for breakfast?
What color tie is your attorney wearing?
I toss those in over 2-3 hours and then ask the question that the deponent could not remember (so conveniently).
I draw two objections - asked and answered and argumentative.
I always ask that we call the judge to get a ruling.
I explain that I've just asked the deponent questions covering many decades about minutia that most people would not recall and the deponent has answered each question without objection from defense counsel. I wish to explore the "memory hole" and how only the fact critical to the case is the ONLY matter that the deponent cannot remember.
Usually the judge gives me a little leeway - but, the record is clear - the deponent's memory is just fine until the fact that will hurt is brought up.
Of course a 5th Amendment objection ends the inquiry (I'm a civil litigator).
The willingness to tolerate the mendacity of poor memory on a daily basis in civil actions puts the lie to this "convenient" ruling.