I might be attacked for my opinion, but here it goes. A lot of people give their horse a little bute on multi-day shows or big clinics to make sure that they do not get painful aches, when they must go from a hard work out and stand in the horrible temporary show stalls. These are healthy horses that did not come to the show injured, the owner simply knows the effects that a box stall can have on a horse after a lot of stress and exercise. Kind of like taking an advil when you start to feel a faint cramp coming in, you can take the medication to prevent a lot of ache later. However, bute is a medication and like all other medications should be used as little as necessary. So yes, I am fine with FEI's decision, as long as it is well monitored.
First of all - bute will not cover up a serious lameness. Second of all, the amount that is allowed is only equivalent to 0.5 - 1 gram a day.
The purpose of bute is to prevent and treat inflammation, not "soreness" or pain. The result of reducing inflammation is that the horse will not be as sore, yes, but the main purpose is to reduce injuries. Inflammation is not only a sign of injury, but the actual act of inflaming will damage some structures not previously injured, especially in the joints. Using Bute, Asprin, Banamine, Cold Hosing and injecting a horse's joints all have the same goal when used therapeutically - to reduce inflammation and thus injury.
Speaking as someone with experience in keeping an older, arthritic horse in competitive shape, this will make the lives of many adult amateurs and juniors on old campaigners so much easier. In order to show without using bute, you have to time farrier trims so that at the time of the show you are at least 2 weeks from either end of a shoeing, you have to schedule the horse's training so that he is not unfit, but not overworked so he is sore and you also have to time your IM/IV adequan/legend so that the horse has recieved shots within 2 days prior to the show. All of this means extra farrier trips ($$), extra vet trips and injections ($$) and possibly missing work to work the horse ($$).
To be able to just take the horse to a show like any other horse and give him some bute, wonderful!
Finally. It is uneducated people that have restricted this from happening 10 years prior. Good riddance, and lets hope that EC and USDF follow suit.