Within the Potterverse it is hard to write a fanfiction with a plot you have in mind with strictly cannon characters. Most people prefer to keep cannon characters cannon and by doing such, OCs are needed to fill the other roles defined by a story.
I feel OCs when developed correctly are a wonderful addition to a story. When working with the Marauder era, in order to completely fill out a story OCs may be needed, because there is really so little information we know about that time. Sometimes it is just good to expand a writer's imagination, also.
All in all, I feel OCs have their place and I thoroughly enjoy them.