Helen, your notion of an acquisitions policy is one I reckon should be widely emulated.
It's various 'relinquishing' policies that libraries have that I don't fully understand. I know that shelf space is finite and tastes change but some policies confound my limited understanding.
Years ago whe I was teaching in Brunswick (Melbourne) our school was told that the local municipal library was giving away everything that had been set up in their basement and we were invited to just take it away. I scored a huge collection of books on folk lore and folk tales and even the full six volumes of Manning Clark's "History of Australia"!
At my local university library I would regard some of their practices as dereliction of duty, as they seem to want to get rid of anything more than a century old, prompting various of us to go on an annual borrowing spree of Darwin's 3rd edition (1865 approx) of "Origin of species", Lyell's 1840 tomes on geology and Bacon's bible of Morris tunes and dances, just so that the statistics include some measure of activity to support the notion they should be retained. So far, we've been successful.
But I do like the program; it gives the same delightful squirm factor as "Mother and Son" and "Kath & Kim"