Turn and slip indicator or turn coordinator is definately usefull, though I prefer the turn coordinator. An attitude indicator even better.
I agree that turn coordinators are preferable in most light areoplanes, where modest angles of bank are used in IMC. Thermalling in gliders, whether IMC or otherwise, involves angles of bank of 40 to 45 degrees where a T&S is a better instrument. My glider has both T&S and a horizon.