wEll, even on an SLR these days you don't need the viewfinder and can use the screen. The benefit for me having mirrorless is the size and weight. My 40D with lenses was my complete carry on allowance. My Olympus is a couple kilos and much smaller overall. It is convenience. However, if I am shooting a wedding I will borrow a friends canon with full prime lenses as the feel of the image and quality is superior. At this point, I don't think you will find many real professionals using a mirrorless for more than a run around.