I've seen several let's plays for this game. It was an interesting idea to develop a first person shooter around bank heists. It's somethign Rockstar would add to GTAV later on. Also, I think none of the bank security would risk their lives by shooting at your crew. It's just me, but most employees are probably trained to cooperate and not to resist.
Yeah, that was probably unrealistic, but something to add to the challenge of the game. Sometimes, realism gets sacrificed for the sake of gameplay and, often, I'm OK with those decisions. For instance, games typically limit the range of shotguns to only have high damage up close. In real life, that limitation doesn't really exist with such weapons.
