As reported here by Peachtears13: https://www.reddit.com/r/AdobeIllustrator/comments/1kei4zn/custom_art_brush_results_in_way_too_many_anchor/

Having a simple path (with minimum amount of points), applying a simple art brush (with minimum amount of points), and then expanding it gives a ton of extra points, hardly needed to define resulting shapes.

It worked MUCH better in CS4... and then became much worse in CS6 (or perhaps in CS5) — take a look at the images attached.

Yes, there are tools to deal with the mess afterwards: native Simplify, plugin VectorFirstAid... but it should not be like this in the first place.

And please, do not 'solve' it by throwing in another hidden Simplify like you did to deal with Variable Strokes!!! This leads to loosing symmetrical point pairs across the stroke, it makes it work slower while editing! Just do better math, optimize each cluster but keep their edge-points intact! Look at the CS4 again, dust off from some old code!