Main Sheet Block Riser

[petej33]

Hi,
Can anyone explain the advantage of raising the main sheet blocks in the stern?

[Bob Gray]

     It gives more room in the cockpit when sailing block to block and when tacking.

[KB [us5219]]

I scratch my head a bit when it comes to this topic...
For me personally, the sail height (Boom height) is determined by many factors  When its blowing like stink lowering the sail seems to both reduce hiking forces on the boat, but also reduce mast bend at the same time (two things that could have opposing results).  But when a mast is bending crazy out of column (say 18"+) not only is it kinder on the mast, but often faster performance can be had decreasing the bend and lowering COE (Center of effort).  Having a riser decreases the range of adjustability you have on sail height, but it will increase headroom under the boom, and allow the boom to sheet closer to centerline (although ever so slightly) at a higher sail height than a boat without a riser.
My current rigging would not work with a riser.  If needed, I let enough sheet out to get my head or shoulder underneath the boom, but generally sliding forward in the cockpit seems to deal with the issue fine.   
Different strokes for different folks.