There have been studies that have shown that better isolation allows you to actually turn the volume *down*, and therefore being better for your long-term hearing. Headphones that do not isolate well force your to increase the volume in order to hear the music and drown out the noise. High SPL is what damages your ears. Anything you can do lower the net SPL is a positive.
I'm afraid I'm not sure what SPL stands for. Sound performance... level?