I assume that you are referring to the blade aerofoil sections, not blade planform. If that is the case then here goes...
The aerodynamics of the helicopter main rotor are extremely complicated. On the advancing side the aerofoils must be able to operate efficiently under a transonic flow regime, that is, the combination of the rotational speed of the rotor and the forward flight speed can produce Mach numbers that are greater than 0.85. This produces pockets of supersonic flow that terminate with a relatively strong shock wave resulting in a large drag penalty amongst other things. On the retreating side the aerofoil must be able to operate at very high incidence at moderate Mach numbers - frequently close to the dynamic stall limit.
On top of this, helicopter aerofoils must be designed so that they do not produce large pitching moments. If they did, then huge control loads would be produced that would require a very strong rotor control system.
Clearly, there are lots of conflicting requirements here. The trusty old NACA0012 symmetrical aerofoil found wide application in early helicopters because it is a moderate thickness aerofoil that combines enough thickness to be able to operate up to about 12 degrees without separating (statically) while being thin enough to avoid developing strong shocks too early. And all of this, with virtually zero pitching moments until those nasty shocks start to appear.
The problem is that the early symmetrical aerofoils are rather dragy, they don't behave very well at higher mach numbers and they have some nasty characteristics at moderate Reynolds numbers. Therefore, in the quest for higher performance machines, we needed to develop aerofoils that delayed the development of shock waves until higher mach numbers and then produced weaker ones. In addition, we needed aerofoils that could operate at higher incidence without separating. Again in all of these regimes we wanted low drag and moments.
So in summary, cambered, or asymmetric aerofoils achieve greatly improved sectional aerodynamic performance, meaning lower drag and higher maximum lift in all of the flow regimes described. This means our modern aerofoils limit the strength of the shockwaves on the advancing blade and delay the onset of dynamic stall on the retreating blade. This makes our helicopters fly faster, further and higher, while vibrating less and being significantly quieter (Reduced transonic noise). However, very often this is at the expense of significant or even large pitching moments.
In most modern rotor designs this problem is countered by using a variety of aerofoils along the span, the BERP 3 rotor uses three different aerofoils for example. This allows you to balance the nose up pitching moments generated by one aerofoil with the nose down pitching moments of another. However, this isn't always enough and so it is sometimes necessary to resort to planform modifications.
I disagree with the earlier remark that 'asymmetric blades are more difficult and expensive to manufacture'. So long as it is a single aerofoil along the span, then the cost is identical to that of a symmetric aerofoil. Things get more complicated when you use more than one aerofoil, as you really need to use composite blades then, but many manufacturers use composite blades for other reasons anyway. Once you have gone composite then you can have quite alot of aerofoils with highly optimised shapes at little additional cost. If you got to complex tip shapes then some cost is added, but not nearly as much as some of the manufacturers would have you believe!
Hope this helps chaps,
CRAN
Sorry Gav,
Didn't read the post properly! The one line answer is...
In general, modern asymmetric helicopter aerofoils produce more lift with less drag, than traditional aerofoils, which make is possible for helicopters to perform better.
...and the other bits...
They don't have a significant effect on the 'ride' on their own, as this has to be viewed from a rotor system point of view. Though they will improve autorational characteristics by virtue of their better lift to drag ratio and ability to delay separation to higher incidences.
Cheers
CRAN