Two other lines I found interesting for Black when I studied 6.Bg5 a while back are 6...e6 7.f4 Qc7 (trendy for a while) and the surprisingly common 6...Nc6, transposing to a Richter-Rauzer where Black has played ...a6 early (but this might easily transpose to major lines of the Richter-Rauzer where ...a6 is included). Why not play a Classical Sicilian instead of the Najdorf? Well one possible reason is a Sozin setup (6.Bc4) is more potent against that than in the Scheveningen (move order for that is commonly with an early ...e6) via the Najdorf move-order. In the Classical, with both Black Kts out, 6.Bc4 Qb6 can be very complicated, and allowing e.g. a Velimirovic Attack by transposing to a Scheveningen with ...Nc6 included is also complicated (and dangerous). Another reason is that people who like to play the Najdorf need to transpose out of it in case of 6.Bc4 e6 (a Scheveningen), anyway (though Black is fairly happy in that case these days since he's not committed to ...Nc6).