Yes, I am with Pete B and Rospa and others on this one, as I have been Observing for the IAM (or now RoadSmart) since 2000, and driving an auto is now 50/50 with manual for Trainees.
The OP was asking about driving an auto, but at least holding the car on the footbrake, is not quite as bad a 'sin' as holding it on the clutch and acccelerator.
However, as mentioned by others, Consideration of (or for) other road users, is one of the headings that is looked at during IAM/RosPa training, and a display of high intensity brake lights especially on a wet night, is hardly 'considerate'. But Cyker list of poor behaviour could all be added to the list of In-considerate and unthinking driving which is always depressing to be a part of. But most of it just thoughtless rather than malicious.
However as for the Prius, co-ordinating the foot parking brake and the accelerator pedal is a great way to hold the car at traffic lights or on a hill, and (if there are enough bars eg 3+ on the HV battery) then slip it into N as well.....combining all aspects of Car Sympathy and awareness of others all together
Again as previously mentioned, holding the ccar on the footbrake alone invites potential problems if shunted from behind, as inertia of the shunt sends the driver back into his seat, and lifts the foot off the footbrake, allowing forward movement into the vehicle in front, or into a stream of crossing traffic.
AS PeteB says, if these aspects of the drive are not habitual, then any Test situation with a Class 1 copper sitting beside, will find out the weak points.
On the other hand, if anybody else would like to chip in and help the situation, by becoming a volunteer ttrainer/observer themselves for the IAM or Rospa, then maybe we could kick a bit mor ***** out there, and improve the UK roads