That happened to me a couple of days ago. Luckily I remembered this item, even though it took me quite a while to get enough BP to buy it xDIt's worth it IMO, especially at times when you pull out a 5 IV Pokemon with the wrong ability
I see it being used as gts bait. I found it to be an upset when I read somewhere that it will not switch to a hidden ability ... to me its useless then, because in my own experience when you start getting to the 5 IV eggs they become more common than the beginning process. So there is more of a chance the next batch of eggs has the ability and the IV's you want... which is superrrrrr less time consuming then amassing 200 bp.
I've never used Ability Capsule before to be honest, so I can't be sure, but… does the item change your ability permanently, or is it just temporary? If it's the former, then yes, Ability Capsule does sound like an extremely useful item. d:
It can be useful since it can save you some time if you have it and need it in that specific moment; when you think about the amount of battles you have to do and the price (and you don't battle that much to save BP or you're lazy) of course you would feel is time consuming. It's about perspective, of course, not being able to change to HA is a drawback, but we have to remember that there are some HA that aren't worth in some pokemon anyway.
I really only think that it would be good for breeding purposes. For the price, using it in a battle...I just couldn't do it. All that work, and poof! At least with breeding you could use it on a multitude of pokemon.