I've tried to include privates with most of my early training trips. Some where game changing, some where "meh". By the way, if you have never done a private with your coach, don't confuse the quality of a private lesson with him answering a question really quick after class. That said, of course they don't substitute for mat time. Your results with privates will vary based on who your taking a private from (their ability to teach a private plus their compatibility with you and your game) as well the point you are in your training.
Personally, I was on my own most of the year so I had a surplus of rolling time, I squeezed in a decent amount of repping what I already know, but got almost no new technique or evaluation of my game by an expert so I was an ideal candidate.
If your interested in doing one, I wouldn't let anyone here talk you out of getting ONE and from their you can gauge how much you feel like it helped vs you being able to use those extra work our to roll some more or something