In the event the isolation in the high frequency transformer between the high voltage side and the low voltage side failed, a well designed PSU would use the ground connection to protect people touching the LED lights. (Usually the ground is wired directly through to the -ve side of the output)
As to your first question: "Can a power surge have a different effect on a led driver when floating?"
If there were a large common mode surge (such as is simulated in High-potential-earth testing) then the presence of an earth connection to the driver might cause it to fail, when an un-earthed driver would have been undamaged. But under such a situation the failure is to protect the devices and the people connected to the low voltage side of the driver. So having it survive, but take out the things downstream is not really a win.