
I would agree with Sawheaties - in breeds with low entries, it can be the only way to keep the classes on and to give the judges enough numbers to progress.
My older male has 41 BOBs at Open Shows. I have only shown twice under a judge that gave him BOB before (Club Open Shows excepted) - once when there were only two present the first time, and another time when the second show was a breed supported show. Neither time did I get BOB, as the competition was quite different to the first time, and it was in a different geographic location. I have frequently entered under a judge twice, but not taken the dog, just to preserve the classes - the only difficulty there is that it may lead the judge to expect a bigger entry to judge than will be the case. It is considered perfectly acceptable to enter a Breed Club Open Show if the judge has given you BOB at another Open show.
As a judge, I have in some cases had my hands on the same dogs several times at various stages of my career (and theirs), and the results have varied depending on the condition of the dog on the day and the level of competition against them on the day. I wouldn't expect them to turn up again and again though, having already had BOB, and I'd be disappointed if they did.
At Championship shows in my breed, it is usually considered perfectly acceptable to show again under a judge who has already given you the ticket if the second time is a Club Show or Crufts, but not otherwise. If there is a change of judge on the day to one who has already awarded a CC to your dog at a normal Ch. Show, it is most sporting to withdraw, with or without a lap of honour.
Edited to say: If I had only had 1sts under a judge, but not BOB, then I would most certainly enter under the same judge again.