The number of idea rates per user enhances their influence on the content promotion algorithm.
The more you rate ideas, the more your take has an effect on what ideas are pushed on the homepage.
It should be a multiplier of 0.0X or something like that, but it's still a good incentive. It also makes sense as the more active users know what's important and more in line with the platform than someone who has just registered.
Ideally, the multiplier could be applied per area or field. For instance, user X has rated 30 ideas in biology, more than anyone else on the platform or in the top five, hence his rating on an idea in the field of biology pushes the content higher up.
Tying this proposition with the one disclosed about badges.
Actually, this might be necessary for when a new influx of users come. As they could overwhelm the rating system and have some mediocre idea get glued to the top of the algorithm.
Promote it/explain it with a taglinelike "craft the future" by showing which ideas are the most important.