1. You're cutting in the line. Most people wont say anything if you do it in person, but I can guarantee you they're thinking about saying something, and in their cars something to the effect of "this is why motorcyclists get themselves killed."
2. You're creating a traffic jam because now there are 3 vehicles and 2 lanes. This is especially problematic when there are multiple bikes that have "filtered."
3. Usually (almost always) the people that do this are riding sport bikes, and sport bikes have a bad reputation in this country for riding recklessly and dangerously. You'll never see someone on a Harley doing this, for example.
This is not common, but here's a recent example in my area of what happens when a motorcycle gets under someone's skin:
http://www.tampabay.com/news/publicsafety/accidents/fhp-driv...