As an official, I do tend to spend more time on courts where there has been trouble or where players have a history of problematic behavior than on courts where things are running smoothly. And of course, if other matches finish and there are only one or two matches on the courts I'm working, I'll be there much more often.
However, frequently you have very few officials for lots of courts. For example, locally we try to do one rover per four courts for any major tournament, but for a variety of reasons (shortage of available officials, TDs running more courts than normal when the schedule falls behind, etc.) sometimes you'll end up with one official working many more. This last year I worked two tournaments where, because of legitimate scheduling reasons, we had 2 or 3 officials for 18 courts. (In one case, each official ended up working at least a 14 hour shift...)
When you have one official working lots of courts like this, the official simply can't stay on one court too long; the other players paid entry fees and are entitled to the services of an official as well, and letting a bad player monopolize an official to the detriment of the other players would also be unfair.
In terms of a "stealth official", I simply think it wouldn't work well. First, it would require tournament directors to hire more officials, which they're already loathe to do. Such a "stealth official" would only be useful if they could overrule calls from *off court*, in many cases from far away through windscreens. To overrule such a call with certainty, the call would have to be monumentally bad, and while such calls do occur, they are still rare. These officials also couldn't conduct the full responsibilities of an official such as preparing courts for play or coming on court to overrule incorrect close calls. Essentially you'd be asking TDs to hire officials to work a full day that could not perform their full responsibilities and would only be useful in pretty rare circumstances - that's simply not going to happen.