The thing i don't get in all the "BPA duh" sort of answers, is exactly how you ascertain what the "best" is? There's no magic crystal ball list that arranges the future best players in any clear order anyway. Especially the further you get into the draft, it becomes a real crapshoot altogether.
That's where for me, positional value plays into BPA...because ultimately, long-term...position plays a big role in the "value" of a player. If we're talking about marginal differences between the quality of players, positions like C and D will have more potential value than a comparable W, and thus be the "better" available prospect.
When you're talking about a substantial drop-off in quality, then sure...draft the higher quality prospect at a less premium position. But really...when is the last time you heard about a team drafting a Winger for "need"?
It just doesn't happen. Nobody really talks about that, because i think intrinsically, most people get that it's not about "positional need" so much as it's about "positional premium". And the idea of positional premium has merit in building a successful team imo.