There is one major benefit that public open beta's have... besides the obvious buzz generated (which in the case of MoH actually backfired LOLOLOL)
that is the Griefing test.
The ONLY way to find griefer flaws in your game is to release it to the public.
Only by letting THOUSANDS and THOUSANDS of people play will you be sure to get the relatively small population of griefer players who spend untold hours trying all kinds of bizzare feats in order to exploit the game for their own advantage.
If you wait until release before letting the public have a first go at it... you will be reacting right out of the gate.... It's MUCH better to have an idea of what people are doing to get unfair advantages, or altogether ruin games BEFORE going gold so that you can patch up the engine BEFORE it's burned on the disc.
Make no mistake... there WILL be people who do everything they can to exploit the system for any and every advantage. It's best to know what they are doing before you go retail and it hurts your most critical moment... the first 2 weeks post launch.