I agree that a backup AI is great, but if the trade off is loosing the only instrument which actually indicates a turn, the cost seems too big to me.
The Wiskey compass gives a turn indication.
A turn and bank indicator won't keep you right side up in a turbulence and a rough flight. By the time you react to a turn in one direction and correct the other, without any other means of keeping wings level, you can be upside down in some airplanes. A 300 or 400 series Cessna with full tip tanks and no additional instrumentation can turn ugly when a pilot gets into a roll oriented pilot induced oscillation on partial panel.
There was a time when a turn and bank was state of the art. That time was a very long time ago. It's a useful instrument, but far less useful than an an attitude indicator when one has to go partial panel.