Problem with any waterproof membrane and essentially an impervious porcelain tile (glazed ceramic can still be a problem, but not as big) is that a modified thinset needs to both dry and to cure. It only has the grout lines to dry from, so it's similar to the effect of using mastic and large tile...it takes excessive amounts of time to be stable. An unmodified thinset doesn't have these problems. The difference in 'stickiness' between a premium unmodified and a modified in total strength isn't all that much, relative to the scheme of things, as either is more than enough in most circumstances. Schluter has given permission to use fast setting modifieds in special circumstances, but these are best left to pros verses the more typical slower DIY'er.