Might have posted a little bit hasty
Recognising APU would be far better...
Since the idea is to taxi WITHOUT the main engine running by means of an electric drive.
Beside that hotel mode seems to have some major disadvantages, as some pointed out: safety issues (prop bake / hot exhaust gases), noise, wind lim. and engine wear.
Nevertheless ATR believes strongly in this system:
Why ATR has not developed an APU for its aircraft?
“ATR offered an innovative alternative to the APU known as the "Hotel Mode". The Hotel Mode concept (i.e. offering air conditioning and electrical power on the ground) was made possible through the application of a propeller brake on the right hand side engine turbine. Thus the weight and added maintenance cost of an APU is spared.
A survey of 20 customers (214 a/c) has been made to gather information about the rate of utilization and the effectiveness of Hotel Mode. The data gathered from ATR Customers have confirmed that Propeller Brake installation is a smart and effective solution welcomed by Airlines due to tangible advantages over APU.