its not the same amount of work
if you write it in a notation program you can save it as midi.
.mid is NOT a rendered midi file
if you want to make it sound nice, you need to put in alot of effort...
all the articulations have to be redone. in a big orchestral piece, thats a whole friggin lot
i usually do it though because i like to take a glympse of how it will sound. :)
and on a personal note, i am not exactly fulfilled with a properly sampled piece either (with properly i mean VSL with perf.tool), actually it only makes me crave even more to hear it live.. :)
edit:
come to think of it, ive spent days just tweaking the sound of the piece in Logic... while it was completely scored. but to be honest, a good sound was sort of inspiration for some parts which made me rewrite some of it.. :)