Rookie.Pilot,
I haven't flown an ATR in almost three years, but I did fly it for two US operators, and I was also a simulator instructor and line check pilot on it.
A bit of background for non-ATR people. The ATR has quite a few speeds that a pilot must bug or observe. They vary among operators in name, but they all follow a script. ATR assumes most pilots fly with the autopilot/flight director most of the time. They have minimum clean speeds for LO BANK (15 degrees maximum) and HI BANK (27.5 degrees maximum). These speeds change when you are flying the airplane in icing conditions. When you select one of the anti-ice systems on (specifically the horn heat), the stall warning thresholds are changed to a lower angle of attack to compensate for the earlier separation of airflow on an icy wing.
We had discussions about LO BANK/HI BANK quite frequently but most pilots and instructors within our organization did the following:
During terminal operations (departure and arrival), airspeed/icing conditions permitting, we used HI BANK. In fact, departing in icing conditions, we would often increase our airspeed a few knots above profile to get to minimum HI BANK speed so we could turn tightly. We flew in very busy airspace, and being a slower airplane, ATC expected us to turn as tight as possible departing and arriving at the hub. Often, after leaving the terminal area, most guys, including myself, would select LO BANK for passenger comfort.
When following instrument approach procedures, speed, configuration, and atmospheric conditions permitting, it is important to use HI BANK if you will be flying the procedure in heading mode. At my last operator, our ATRs didn't have GPS for navigation. So our pilots would fly VOR and NDB approaches in HDG mode (NAV mode didn't work very well with the VORs in our route structure). Keep in mind, the difference in turn radius between 15 degrees of bank (LO BANK) and 27.5 degrees of bank is close to double, so you can see the importance of using HI BANK conditions permitting in this situation to ensure that procedure turns, holding pattern entries, etc. worked out well.
As far as the safety aspects of using LO BANK versus HI BANK, I'll make the following points. True, stall speed is higher with a loaded wing, i.e., in a turn. Which is precisely the reason ATR provides minimum HI BANK speeds for maneuvering protection in icing and non-icing conditions. So this is accounted for except in the case of severe icing, in which case, 1 g and 0 bank angle might be more than the wing can handle anyway. As far as an engine failure in a turn, I have given and received many of these during turns in HI BANK in the simulator. It's never fun (I guess that depends which seat you're in that day), and it might be more manageable with a lower bank angle, but keep in mind that we are professional pilots. I am pretty sure the outcome of each of your attempts, while hairy, was successful?
So as far as it being mandatory, I am not sure what your airline and regulatory authorities say about it, but my biggest concern about flying almost exclusively in LO BANK would be turn radii when being radar vectored at busy airports and when conducting instrument approach procedures using HDG mode.