Sad to read that so many people - some who are some of the most solid posters on these boards, let this happen. There can't be any 'occasional' - the rules are simple and there for a reason: if there's no umpire/linespeople, the player calls his side and if it's not CLEARLY out, it's in. When people deviate from this, bad things happen; players start thinking it's acceptable and when they play other people who know the rules they get upset.
What are you going to do when you play one of these guys and you crush a return winner and he decides that his serve was out?
There are many instances in life that need discretion - line calling in tennis isn't one of them.
"I jump in it!" Russian Billionaire, DirecTV Commercial...