It's so they can use one less linesperson. The person calling the middle service line usually only has to look at the serve then do nothing. Under the MC system (and many Challengers etc), that linesperson now calls the centre service line and then moves to the side line afterwards and calls that line for the remainder of the rally. It makes sense because the linesperson will always be able to move to the side line in time but, yes, it is a bit distracting.
I kinda figured as much, in MONTE CARLO of all places they are skimping on linesmen.
During one match 2 lines men equi-distance apart DARTED repeatedly after the serve just as the returner wwould be preparing to strike the ball!
That's putting it mildly!
Thank you for responding though!