Can someone be so kind as to point out why/when to use a scimitar shaped prop over other shapes..??

Thanks,
Pete

They are more efficient, and usually quieter. They are good for pattern/sport flying, but not good for scale planes (primarily due to the scale look--I don't like the look of a gray APC prop on the front my warbird). However, the black MAS scimitar props might be ok for scale appearance....

i run them on my 50 cc gasser and have alot better pull over the paint sticks called zinger

Its mainly fashion, the scimitar profile made with stiff material has some advantages. If someone made square tip props out of carbon they would work well too, but nobody would buy them. Wooden scimitar props are still very noisy, especially at the low blade angles everyone is using.