movie oddities that make no sense

After the initial skirmish, it takes the aliens a long time to crawl into the ventilation shafts so they can infiltrate the colony HQ from above. So the logical question is why didn t they just try to bash the doors in? How hard could it have been for them, really? According to Cameron s commentary they did destroy the doors but the scene ended up on the cutting room floor due to time constraints. Excuses, excuses.