That's exactly it. If you have a 1.6KHz board like Seppuku... and use broadcasts to send to the ESCs (and they pull out the data for their index), you've got about 107 bit times-- not bad, 17% of the bus bandwidth used. But then when you want to do something new and useful, like get actuator feedback (say, commutation count from each ESC each update cycle, or current reading)... suddenly in addition to those 1600 messages you need 6400 more. 17*5 = 85%, so theoretically you've not used it all, but it's getting awfully close.
And this is just for quadcopters, not hexes or octos.
For attitude, sure, CAN is fast enough, but so is piddly slow 57.6 kbit/sec async serial. 5760 bytes/second. StabilizationDesired (which communicates a bit more than just a desired attitude-- also desired modes on each axis etc) is 20 bytes, plus 9 bytes of UAVTalk overhead, easily does 100Hz of attitude updates using only half of one direction of the link. (And 100Hz is plenty, because there's about 20Hz of attitude-loop bandwidth on even really aggressive things).