Of course VSL comes closer than anyone ever got in sampling realism. The introduction of the legato recordings, playable in realtime, makes it possible to write, play and record the legato line in your head without hardly any tweaking afterwards. You get all the little clicks and glisses of the real instruments, but still... a computer doesn't need to breath, doesn't know how to phrase, doesn't place natural accents and crescendi and decrescendi by itself.
Even with 1.000 zillion samples and controller tools, you still have to make little compromises and manual tweaks, always, though of course much less than with a basic set of just a sustain and a staccato sample. [;)]
Samples just aren't real players, and I hope they never will be! [:)]
As for the "writing for samples" - if you ever caught yourself scrolling through your midi tracks, finding the right kind of sound while fiddling on the keys... that's what I mean with "writing for samples". Or simplifying/changing a melodic line to make it more suited for the samples performing it. Things like that.
The big question is in what your end goal is: a mockup with an ultimate amount of realism, or a mockup with an acceptable amount of realism. Both goals are legitimite, of course, but both have their own limited amount of writing freedom.
Maarten