[concam]

>What's the point of investing in bike infrastructure if (some) bikers refuse to use it?

The same reason we might invest in traffic lights or stop signs even though cars regularly ignore them.

[smileysteve]

This is a bad example because it implies that the bikes aren't observing traffic laws.

A more analogous would be that some cars choose to go off roading, or spend most of their time in driveways or parking lots - or some only drive to car shows. A much more accurate analogy would be cars that only drive in one lane and never use the left lane for passing.