Subframe - Rust prevention?

[elebub]

In the process of replacing the rear subframe due to the usual rust rotting the frame from the inside to the outside, near the bush. Going to give the new crossmember cavities the full Bilt-Hamber treatment but it occurs to me that the two holes where the bushes are welded to the frame and the circular holes at the diff end are the main culprits. What function do they perform? Just seems to be a perfect water trap.
Is there any advantage in sealing the holes, or am I just going to create a vapour trap?

[whipsey]

fill with waxoil possibly?

[Brianmoooore]

In the process of replacing the rear subframe due to the usual rust rotting the frame from the inside to the outside,

Do some of you live in salt mines or something? I've just been looking at pics of the rust on illmonkey's iS, and now a post about subframe rust! I've never seen a subframe with paintwork that looks anything other than new, except for a little fraying around the hole that the prop goes though!