I believe FR is important because it shows SPL as a function of frequency. A properly measured headphone, should give an indication of the neutrality of a headphone, or at least an indication of what exactly you are buying besides the nice box + way cool build materials and design.
Limitations to what FR tells you is relative to the last part of the chain: your ears and brain. What I personally try to do, is see a FR graph (CDS, impulse response - and from as many web sites as I can) and try to match what I hear with what I see and decide what I want.
Another limitation that has been measured elsewhere is the manufacturing variations of the same headphone. If a manufacturer makes a HP with a certain FR that you like after matching it to your experience, and suddenly changes the build of the HP so that the FR of the previously measured models no longer match what you are about to buy, then yes FR no longer applies... But that is a flaw in the manufacturer's process. Such a change should call for a different naming of the HP so that consumer get what they pay for.