the Boeing fmc after recent software update , all have geometric path. and you are absolutely right. many a pilot don't alter and program the pages sufficiently to get accurate info. remember, garbage in garbage out !!! the key to using descent forecast page is to input all relevant information like TAI, forecast wind component at 3 different levels,isa dev and airport QNH, along with transition level.
remember that if you don't input the transition level, this info is ignored in calculating your path. same goes true for all other inputs. hence not entering twc will eventually bring you too high as also ignoring TAI.
the key to using fmc is to use all relevant info, it need not be absolutely correct but only ballpark, and it will give much better results than otherwise.