In my opinion, the secondary character voice sameness serves two purposes:
1) budget: leaves more money for foreground activities (audio version of "blob people"
2) art: people in the background of life all sound alike unless they're about to step forward to stage center area
I'd say its a lot of both. L*S isn't a "Low budget" production. Please reserve that for things like "cabbage love" or even some favorites of mine (Petopetosan, Lamune). L*S is a *lower* budget production than something like Kanon or Clannad, I'll surmise. But then, the art style of the comic permits that anyway (this isn't hyper-realistic style L*S, eh?).
They save their money shots for specific moments and it shows what they might have done with an infinite budget.