I refuse to use fans that have a plastic frame (not the fan itself, but the part that wraps the motor). Seems like you are putting a blanket on the motor that would trap almost as much heat as the fan could chase off.

I am also in the camp that you should not do your setup to depend on the fan to keep the motor cool enough. A fan is fine, but it should not be critical. And in a buggy vs short course chassis the body is much tighter around things not letting as much natural air flow happen, so if you do not have air coming into the body in the first place you are just recycling hot air (except of course for rear motor 2wd). And in those holes are a bigger deal than a fan to me.