most of the "stuff" on the spokes is
brake pad compound, you need to clean it, and yes rust will attach the spokes too.
the newer the model the worst the are.(cheaper parts supplier)
about the broken spokes that is a new fenomenom,is becoming a epidemy for the last 2/3 years.
I make you a bet the spokes that break are the ones that comes from the final drive side and go forward(as oppose to backwards)
and ussually they break in pairs(don't ask I don't know)
a friend of mine and me , made a paper about wheel repair if you want e-mail me and I send you a link.
the BMW spoke wheels are the strongest wheel you can find ,but they do need to be "tune" and keep with equal tension,so all the spokes share the same responsability...