True, the torque requirements for servo speed haven't changed. For trainers and basic sport planes, no one "needs" 6v servo performance.

However...

In the days when 4 cell NiCd packs became standard, we had analog servos and 72mhz radios. Analog servos weren't power hungry, and 72mhz radios were very tolerant of low voltage situations. Today, nearly everyone flies 2.4ghz, quite a few the Spektrum brand. Spektrum equipped planes will often crash just from a split second drop below 3.5v. Analog servos probably won't cause that with a good 4.8v pack, but if the pack is marginal and the pilot is really working the sticks, it can definitely happen. Digital servos will routinely cause a 1v and more voltage drop when they start demanding power, so it becomes good practice to use 6v batteries and be done with it. Very few currently manufactured servos are 4.8v only, so there isn't much reason not to go ahead and use 6v.