I think it is in certain circumstances. I have been around horses for years dealing with injuries and will give a horse of my own bute short term if injury is something minor that I can deal with myself. I also have an arthritic mare who I occasionally give bute to to keep her comfortable. .....However I have come across a lot of people who assume diagnosis (especially with laminitis!) and give their horses bute in significant quantities for a while without consulting the vet. This is wrong in my opinion as there are so many other things it could be ie. abcess/infection.
I personally think it is. Especially by people who have not been told to use it for a specific reason by the vet. If we could diagnose what was wrong with our horse medically then we wouldn't need a vet. It is a perscribed drug as far as I know and as such should be only used under a vets supervision.