Honestly the addition of Biehn and Henriksen, et al, was so late in the day I suspect it was all just a marketing effort to try to help mask the fact that they were stuck with what they knew was going to be a real stinker of a game.
Therefore getting original cast involved was a way to hopefully distract the fans and better give the illusion of this was going to be an a-grade release title, when it clearly was never going to turn out to be a-grade no matter what happened.
funny you should mention that, since the Wiki page said this too:
Gearbox’s CEO Randy Pitchford, speaking exclusively to Official PlayStation Magazine explained that the release delays were decided to include several actors from the 1986 Aliens film in the game. According to Pitchford, actors Michael Biehn (Corporal Dwayne Hicks), Mark Rolston (Smartgun Operator Mark Drake) and Al Matthews (Sergeant Al Apone) reprised their roles, while William Hope (Lieutenant Gorman) and Lance Henriksen (android "Bishop 341-B") will play two brand new characters (Commander T. Shannon and android "Rook" respectively). A marine character from Aliens called Wierzbowski will also appear, but not voiced by the original actor.
So Yes, the voices were probably distraction, completely. Even lying that the delays were because of the voices and not because the game was not ready.