I would like to hear from the experienced about this.

I am putting forth a hypothetical situation where you know & trust the bitch's owner, that the pups would have good homes and be returned to the breeder if needed, and also where BOTH dogs are excellent specimens of their breeds, confirmed by show and working results, with all health testing complete.

>where you know & trust the bitch's owner

Even if you know and trust the bitches owner, do not take it for granted that they will act/do as you expect them to once the litter is on the ground. Many a 'friendship' has been destroyed when known and trusted 'friends' have then gone on to do something that later horrifies you. Don't take anyone's 'word' that the information they provide you with about a dogs health test results and even 'registration' are fact. Always ask to see the documentation. Many a stud dog owner has been caught out by 'trusting' a breeder's word, only to later find out that the bitch is endorsed or test results are not what was implied. You need to do your homework and watch your back no matter who they are :) That way you do not get any nasty shocks later.

Obviously if the dog has unusual or rare lines, it is most useful

As for how it affects the dog some change not one iota, some become more bitch aware.  In my experience young dogs can be more of a pain, and older ones know their stuff so only get worked up over bitches at their peak of fertility.

As they aren't likely to meet any of those regularly, unless living with entire bitches it generally is not a problem.

Some more dominant or conversely less confident males may have issues with other males.

Very good points re people not turning out how you expect! I have just seen one strong friendship break down completely between breeder & buyer when pup turned out to have a health issue which breeder doesn't accept as truth. So it can happen to anyone no matter how well you think you know them.