I think it's got to do more with what would benefit Sony as opposed to what would benefit the consumer . The PS3 is marketed as a games console/media centre and keyboard/mouse functionality in games would compromise that , a significant of % of the consumer doesn't recognise that the PS3 is simply a gimped PC and Sony would like to keep it that way.
However if there is enough demand for Keyboard/Mouse support then Sony might have to oblige . I'd hate to play against Keyboard/Mouse users online though that would suck if I used my DS3 , a standardised control scheme for everyone is better imo.