Thanks, you're probably correct.. as Hasselman said above, probably a cost saving exercise not blanking it off.Often these threaded bosses serve a purpose in the factory, on the assembly line. Typically, lines and robots will lift and carry via connecting to a boss such as this.
Can't say for certain that's the reason for this particular one, but it's a hunch. You'll find a few of these around your 310 engine, and on many engines.
As for leaving them open, BMW sees no concern.