More and more helicopters are going the way of asymmetric aerofoil sections for a number of reasons. I fly the Westlands Lynx amongst others, for example, which has the BERP blade (as does the EH/US101) and has a paddle tip and an asymmetric section along its length. You do have the problem of movement of centre of pressure but it is removed by the use of hydraulics (it is not flyable without hydraulic assistance). Most large modern aircraft are unflyable without hydraulics. The asymmetric aerofoil give a much better figure of merit than a symmetriacl one. The BERP tip is designed to reduce retreating blade stall (the notch re-energises the flow over the tip), improve compressibility problems on the advancing side at high speed (using a thin aerofoil). The area of the tip is spread equally for and aft off the main blade to reduce CofP movement problems.