With all the doubt you're having, why not just install it and stop worrying. It comes with BB's, and these external BB's are SO easy to work on, so why not just install it according to it's design and instructions???

Multiple times I've been out in the rain and forgotten to drain the water when I get home. The next time I ride, while pushing my bike upright through the door I'll be pouring water out of the drain holes at the rear of my chainstays. As FBinNY said, water comes in multiple ways, and WILL get to the BB. Probably more water than you think. If it was my own bike and I'd left it off for some specific reason, I'd definitely take the 20-30 minutes to go back and install it properly.

-Jeremy