Mine are also frozen open - which is just fine by me since it never gets cold enough to worry about needing the engine choked up too much when starting. I also don't ever plan on driving in the rain, but if I must I'm not going to worry about it. As Roy, Jeff, and David have mentioned, it would take a LOT of water getting sucked in to be enough of a threat for engine damage. Aside from the drain holes doing their job, the air filter will saturate and cause performance issues long before there's any danger of the engine seizing up.
It would also probably take more than a teaspoon to really FUBAR an engine, but David's right that it doesn't take much to do so. I watched a Chevy Beretta suck in its fill at a low water crossing after a storm one day, and we were able to get it running again a few minutes later after pulling the plugs and evacuating the water (shot it out the plug holes about 15 feet when we cranked the engine). After drying out the air filter and replacing the plugs, the car reluctantly fired up and smoothed out after a few minutes... back to normal.
Oddly enough, there are actually water injection systems out there that shoot a mist of cool water straight down the throat, the theory being it will cool the air and effectively condense it for more power (with more fuel, of course).