Well, some experiences involve crowds--that is part of the experience. We were in Spain a few years ago for Semana Santa in Sevilla and Granada, and the crowds, noise, and general commotion all added to that experience. A festival with no crowds would be like going to a sporting event and being the only people in the stands--it would just not be as exciting for most of us.
Of course, some people don't like crowds no matter what, so it's a no-brainer for them--just don't go. But, again, what would a festival be without crowds? Not very festive, I think.
Now the impact of the crowds on your ability to see the most popular sights in Barcelona is a separate issue. I don't know how many people from out of town would be drawn to the festival. The more of them there are, I guess the bigger the impact on the major tourist sights. On the other hand, if the festival attracts mostly locals, then it shouldn't be much of an issue.
We're going to Venice in early September, and specifically picked the the weekend of the big Regatta Storica so we can experience the celebration, including the crowds. Others avoid such events like the plague.