Putting a snail screen on your drain keeps snails out as well. People have been running single drains for a long time and, I agree... if you clean your tank and pipes on occasion you should not have to worry about it.
Sure Denny, a second drain is nice... but not necessary. It comes down to personal choice I believe. Single drain tanks would not be out there if they did not work. Having an emergency drain is something easy to do if, say you build a 200 gallon custom and they factor the cut into it already.
I almost added a second... but after soliciting advice from several reefers and staff at a few LFS, I cam to the conclusion to leave my single drain and clean it and the screen.
Maybe its famous last words.......