there are few reasons, and those are mainly due to painting, not the strenght...Silica - speeds up the setting of the lime hence shrinks your painting time, colors "slide" when painting on silica. (it is also part of the reason why it is not quite possible to work on dolomitic - magnesium lime)
secondly, althow it is "edgy" and sharp, high calcium lime does not like to stick to it (meaning layers)
Garry, i know that you use regular lime (type S - with a lot of magnesium) so you do not have "sticking problem", however magnesium + silica give tenfold of the problems in painting and after all it is about the painting - isn't it?
more later, was out of the door...