I think we have been through all arguments before. So for only one toss a series there is a problem. According to playing conditions pitch can't be altered except in very specific and approved ways after the toss. By having only one toss you are either saying that pitches should not be worked on after the first day of first test in the series. If not that then you are basically giving huge advantage to the home side. For the games where they get to pick they get very lopsided pitches where winning toss will be of greater advantage. For the games where the opposition gets to pick they get a balanced pitch. Considering captains openly whine about pitch conditions, and all over the world, it would be no surprise if this thing happens regularly.