A professional photographer is going to be able to capture you in the moment of being your character, not you stressing to run back into the shot after hitting the timer. Not to mention, they're a professional! The skill these guys have with a camera is going to make you look even better.
Allow me to demonstrate my point. Here is a picture of me taken by my sister at Anime Weekend Atlanta in my first cosplay, Shura from Blue Exorcist with another cosplayer.
It's not perfect or the best Shura cosplay. Heck, I know I'm not built the same way as she is. The photo is good, but that's about it.
Now here's the same costume, same girl, but with a professional.
* Professional photography taken by Pixlette Photography.