Again, its more theoretical than anything else, but the engine bay in the ST is incredibly cramped, meaning that it gets very hot in there. Engines need plenty of cool air to work at their best, so if you've got a cone filter without a decent heat shield (or cold-air feed at the very least) then its just gonna suck in all the hot air from the engine bay and (potentially) lose you power. This isn't normally gonna cause you too much of problem in the British winter but its a potential concern for when the few weeks of summer come round again, or if you live in a slightly warmer climate.

I know of plenty of people who have now fitted stuff like the K&N 57i without any real complaint though, but if I was looking to get a proper induction kit then I'd be inclined to put my hand in my pocket and get something enclosed like the Pipercross Viper or BMC CDA. The K&N Typhoon should be out before much longer too.