Filtering back was a traditional part of cask beer management for many years. These days most brewers big and small do not encourage the practice,and for obvious reasons. Beer once exposed to the open air through pulling off is exposed to all kinds of bacteria and yeasts. Re-introducing this back into a cask can and does run the risk of contaminating the whole cask. Before the purists come on and say they have been filtering back for years without any problem I will say that filtering back can be done, but given the risks to beer quality is it worth it.
In the past unscrupulous landlords used to filter back cask bottoms and general slops, not very nice really and totally unacceptable these days. Cask ale looked after properly should not create the need to pull off great quantities of beer. Our own engineers do not recommend filtering back.