Tony, if you're going to make crap resolutions about game design (of which you are not a professional), please don't use my old avatar to do it.
Half-Life isn't about realism. It's about making interesting set-pieces for the player to travel through. Each presents a question which may have one or multiple answers. Each is designed to either teach new concepts or reinforce concepts previously learned. Listening to any Valve developer commentary will tell you this.
Barrels are designed to be a recognisable element of interaction. Players learn and understand the benefits of the barrels, and hence they feel good when they make use of them after they learn the best strategy of waiting for enemies to be close to the barrels. Part of the game's questions revolve around a simple gameplay element, the barrels, and how to make best use of them.
Please never become a professional game designer. I'm sorry you have some kind of fear of barrels. There's also hundreds of enemies in Half-Life, are you going to cry about that too?