Blending with gamma 1.0 will be slower, and can lose precision (quantization).
Blending with gamma 1.0 will make existing documents, or documents from other users look different.
Most users will not need to change the settings.
Some users who are working with third party software that only works in gamma 1.0 may need to change the settings, and some researchers may need to change the settings for their work.
Blending text using gamma 1.0 will not look correct.
Text antialiasing is still more of an art than a science. (and this is why there is still a setting for it)
The gamma blending setting for text in CS6 will result in glyphs appearing the same weight when light on dark and dark on light, and at different positions relative to the pixel grid. Gamma 2.2 makes dark on light too fat and light on dark too thin. Gamma 1.0 makes dark on light too thin and light on dark too fat. Both extremes make glyphs appear to change shape when positioned at different phases relative to the pixel grid.