In human medicine, out of date stuff is sent off to 'third world countries' to be used. If it's OK to be used then it's OK, whether or not it's past it's use-by date so why can't we use it here? and if it's not OK to use here then why send it on to other people? I'm sure there's a word for that but I can't think of it.
Some meds we keep beyond the use-by date, for example Calciject, but not AntiBs. Our vet will sell single doses of metacam or an unusual AntiB as we don't use much, but we always have a bottle for lambing (haven't used any for years so quite a waste).
The main problem I can see in keeping out of date meds is the way farmers tend to use them. Some may not use a clean syringe and needle with each withdrawal from a bottle, or clean the top, thus contaminating the drug. Our neighbour gave us an AntiB dose during F&M in an emergency which was very kind of him. However, the syringe and needle were totally filthy - he said 'it's an antibiotic so it doesn't matter'
Total stuff and nonsense of course.
So if you know your sterile technique is flawless then it's probably OK to keep bottles a bit beyond their use-by date in some circumstances, but if not then don't do it. You are risking making your animal sicker, of causing an abscess at the injection site, and/or using a drug which is no longer effective.