I think arms would be perfect if there was easier to solder packages. (even if they came on a basic board with just breakout pins.)
I currently have an eval board with an arm7 that features:-
16 / 32 bit architecture
62kb flash / 8kb sram
16 x 12 bit adc pins
4 x DAC outputs
on chip temperature sensor
on chip voltage comparator
UART
2 x I2C / SPI
40 x GPIO
4 x timers
3 x 16 bit pwm (3 phase)
External memory interface
JTag
I really dont think that avr's or pic's offer much more, its just that ARM's are difficult to work with on protoboards. Most development utilities like kiel offer a lot of examples
They always seem to be more useful for computing applications and less for robotics apps . . .
They do have the usual peripherals to use for robotics, but with the additional power they provide could push embedded robotics forwards by allowing things like onboard image manipulation for camera apps, its once again just finding a more friendly way to use them...