Another reason Drizzt & co. are out of place because they doen't belong in the location you meet them in (they should be up in Mithril Hall or Icewind Dale). In fact, I believe the latest Drizzt series, Hunter's Blade, takes place during roughly the same time period as the Bhaalspawn saga - though of course Bioware don't have time machines, so they had no way of knowing that
.
As for Elminster, since he can teleport himself virtually wherever he likes, he's somewhat less out of place
. He also owns a house in Beregost, so he at least has ties to the region.
Other pnp characters appearing in the game, such as the Council of Six (two members of which appear in unmodded BG2), Renal Bloodscalp, Lilith Lurraxol, Skarmaen Alibakkar, and Ribald Barterman are not out of place in BG2 because they
do belong in the locations you meet them in.