This hasn't changed, in our experience, since priority was made more popular. The displayed time is usually a fairly accurate indicator.
They generally get the times right, yes. But when all the queue times say 2 hours, you haven't really got a choice, but to queue. For us, priority passes have changed the queues.
Just one example, is Toadie's Cars at chessington. Main queue, can be up to an hour. For a tiny kids ride. We saw the queue, thought it looked really small (it was) and decided to join. But as only 2-4 people at a time (i.e one family) and the cars moving very slowly, then every time a person walked up the fast track, they were next on. This kept happening, to the point the main queue did not move at all. Lots of people left, and were complaining it was taking so long.
At thorpe park, some of the fast passes don't go via the exit, they go by joining the main queue, and anyone who is about to be 20 minutes away from queuing, is now 40 minutes away from queuing because a group of fast tracks have joined it. So it does affect the main queue, whether it tells you that or not, if the fast passes weren't there, your queue time would be 20 minutes better (for example).
I've had the Q bots at Legoland, and had the best day ever, and have also had no Q bots, and not got on one single ride all day because we couldn't queue for over 2 hours for a piddly little 2 minute ride.
I can see both sides.
At Paultons Park, which is an amazing theme park, better than Legoland/Chessington any day. There are no fast passes there, and I realise that the visitors there are not as many as at Legoland (but it's getting bigger) and the queue times are amazing. Even on a packed day in Summer.