I don't think anyone knows what it's for or even whether it's essential. One time we discussed it years ago the only thing I could think of was it's something to do with venting the ozone created by the points. The air flow through the front cover would tend to draw air out through the tube but on the other hand how would a flow of air get in to the canister as that's the only vent ?
It's definitely not intended for the points wire as that is routed outside the can via the condenser termination.
As for replacing it, any rigid plastic tube of the same diameter would probably do the job.
Edit:
Here's something even more curious. I have the 78 version of the official BMW workshop manual. It contains half a dozen pictures of the bean can installation and not only is the tube not shown but the can doesn't even have the grommeted hole where the tube fits. Pictures in the riders manual don't show the tube or the hole either. Perhaps there was some problem with the very early production machines that the tube is intended to address which would suggest it does serve a purpose.
