Honestly, I've never used them... but the claim is that by allowing for a higher pressure release point, the boiling point of the fluid is therefore also higher (remember from physics that pv = nrt... higher 'p' means higher 't')... that means the fluid can continue cooling the engine at higher temperatures before it boils... the rest of the cooling system needs to also accomodate these higher temps... i suspect nissan spec'd the cap appropriately to allow for sufficiently high psi... also, not hearing of any excessive water temps, mostly oil temps...
I'm interested to hear other opinions, but my gut tells me this is not needed
reference
PIAA High-Performance Radiator Cap :: Sigma Automotive