I'm starting to think the timing makes perfect sense though, mainly because having it during the Comic-Con weekend enabled them to build a lot of buzz and hype surrounding Avengers, which I think was definitely their intention. Also, there is ALWAYS going to be competition at this time of the year, there are always big movies in the summer period. Releasing it earlier, there still would have been competition and releasing it later, whilst there would be less competition, it's also not the prime time for people to see movies, hence the reason the summer period is chocablock with films in the first place.
And, whether you like it or not, taking viewers away from HP IS an achievement, considering that is the 8th film in a long running franchise, and this is a new film for a character people haven't seen before. For a new franchise to have that effect on a long running franchise is a huge achievement.