I agree with Marg that for a rental your tenants are not going to look after it anyway.
However, for sale, or for getting it ready to rent, Shannon Lush has the answer for this. You rub some type of essential oil on the surface of the glass, which "fills" the teensy holes that cause the pitting that makes it look like soap scum. As you say it could be glass cancer caused by teeny little holes in the surface of the glass. Once the oil fills the miniscule holes the glass looks great for a couple of months, when you reapply the essential oil again.
Obviously your tenants will not bother with this, but it will at least make your glass look good for renting, and particularly for selling.
I'm sorry I cannot tell you what oil it is, but she has a couple of books out and you could either check the answer out in a bookshop or try her website.
PS. Did a quick google and found this page - scroll down and you will find it is Goanna Oil.
http://today.ninemsn.com.au/article.aspx?id=229019