Although this is probably dependent on your local laws and how much you abide by those laws, as I know lane filtering and such is illegal in a lot of the US.
But yeah, this is more of a special case than generally applicable to most places. On your typical suburban commute you can definitely save time by filtering between rows of parked cars at lights. That just doesn't work here very well.
IIRC only California allows lane splitting. So this is a legal issue as well.
And you're forgetting the double parking, bad parking, standing cars, building construction, street construction, etc. Streets of Manhattan are chaotic. Trust me when I say that there is often not any room for motorcyclists to filter past cars, especially not when bicycle lanes are present.
It being legal or not isn't the issue; it's not possible.