My two cents worth. From a lot of basic IFR training demonstrations in a helicopter simulator (actually FTD) that had two different models - one had a turn and slip and the other a second artificial horizon.
When the main artificial horizon was failed in IMC conditions, the survival rate for trained, experienced IFR pilots who had only the turn and slip indicator was pretty low. Everyone who had the second artificial horizon did OK.
Given that you can make an iPhone an attitude indicator, why are we sticking with the turn and slip (or turn coordinator)?