I think it's almost never a scenario of pure BPA vs pure need. That would be something crazy like a team in great long-term shape in net, but taking a goalie anyway if he's the highest player still on their board. Or passing on an elite center to take an average defenseman due to positional need.
In practice, it's a matter of using both components, but how far on either side of the middle that you drift for a given pick. Like, if they rate guys 1-100 (100 being the perfect player), and at #6, there's an 82 winger (say Tkachuk) and a 79 defenseman (say Bouchard)... Does the bigger need on D justify the slightly lower rating? Would it still justify something like 86 (maybe Zadina) vs 79? Each team not only has their unique board, but their unique balance of BPA vs need.
Personally, I think prospects 3-10 are more or less all one tier of talent, so I'd happily take either Dobson or Bouchard over either Zadina or Tkachuk (or Kotkaniemi or Wahlstrom, for that matter, who I think are right there as well). But that's "my board", not theirs.
The good news is (in my opinion) there doesn't seem to be a really lousy choice available at 6. I would definitely hold my breath about Boqvist's health or Hughes' size if either was the pick, and I like other skill sets better than that of Tkachuk, but none of those prospects look like an imminent disaster.
In the bigger picture, I just hope that none of Detroit's first 4 picks leave everybody scratching their heads, and that at least 2 of those picks turn into very good NHL players. Fingers crossed for a very good defenseman, but we'll see.