Have you played L4D CDF?

If you get surrounded in the middle of a field you're as good as dead. Even though that doesn't happen very much... but you want less openings. If you make a stand on a mini building on Mercy Hospital you're screwed, but if you stay in the "safe house" you have a good chance of living until you bolt for the chopper or a tank comes barging in. A corner is a much safer bet, because you can't get flanked.

And the part with the elevator, with infected on 3 sides. That's not even a good one. You want 2 sides at the most.