I think the other thing that hasn't been brought up yet is, how many times does one or two close line calls determine a match? It's pretty rare.
When I play a match, I can usually tell if the ball I hit was out or in, 99% of the time. There are a few that I have no clue, and have to rely on my opponent to make the correct (and honest) call. If he decides to cheat me on all of them (which I've never yet run into)....? Well, even so, most matches aren't SO close that those calls will make a win into a loss, or vice versa. And if he's honest, and just misses a few calls...well, he'll likely honestly miss a couple of calls in your favour, too, so theoretically it should even out.
And if there is a match that hinges on a couple of line calls....well, if the match is important enough to really care about in the grand scheme of life, then there'd be linesmen and umpires present, right?