It is, a lot of people pitch a new batch onto the trub of an old batch for the flavour that it gives (eg. fermenting a red wine then pitching a cider onto the yeast cake will give you a red-winey tasting cider). I don't bother with it, here the yeast is so inexpensive that I would rather just use a new lot than bother with dealing with off flavours that may arise from stressed yeast.