In my opinion, it's both. There have been features implemented directly because of a thread in the Suggestions forum. There's also a list on the Dev server of possible future features, from threads in the Suggestions forum.
But there's also going to always be people who have ideas for things that won't be implemented. But they're going to post them anyway. They even post them when they know they're on the restricted list. If we didn't have the Suggestions forum, those would appear somewhere else, and no amount of mouse-happy moderators would keep things in check.
So having a Suggestions forum means that all the badly-conceived ideas have somewhere to go - and usually be shot down by their peers - but it also means that good ideas can be proposed, and developed, by those who otherwise might not be involved in game development.
____________"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."
Only fools and children dream of heroes.