To be fair, it was very early in the first set. It's not like they started questioning our line calls after they were down 0-6; 0-5 or something like that. At 2-1 in the first set, I doubt anyone would have predicted such a clear cut win (at least scoreline-wise).
Regarding the challenges in the pro matches: I agree that most of the time they are used strategically in the off chance a ball may have just clipped the line on an important point. I think a better gauge though, is seeing how the lines people can call certain balls out, that a replay will show hit right smack on the line...and to think, that's the only thing they're supposed to do is watch the lines.
Good example: I was actually just watching the Miami match between Ferrer and Fognini. At 6-1; 1-0 and 15-0 Fognini challenges a call that was called wide, and the replay showed that 95% of the ball hit the sideline.