I've ridden where it is legal and split where it is illegal.
From my perspective, there are very few downsides to lane splitting. California, where it is not illegal (subtle difference), it reduces congestion a small amount, and allows people on motorcycles to make progress when cagers can't. According to CHP, splitting accidents comprise a very, very small percentage of all accidents.
Of course, California has some of the better freeways out there with both wide lanes and plenty of HOV lanes, making the whole thing easier. T'would be a different story in states where there freeways tend to be more narrow in and around cities.
I'm currently in the Philadelphia area and the very thought of splitting somewhere like I-76 when things are congested would be terrifying.
Sometimes you just gotta!