Of course leader and tippet line matter, just as rod weight matters. All else being equal, heavier the rod and/or heavier the tippet/leader, the more pressure you can put on the fish. There are those who believe that the lighter rod is better, but what it comes down to is they prefer the fight, and there really is not point in an "intelligent" discussion because they've already made up their mind. Same is true for the tippet/leader debate, although here they debate is clouded by the desire to actually catch a fish, as opposed to just landing one. One distinction is if you use too light a leader/tippet, the fish can get away, whereas if you use too light a rod, you increase the fatality rate of the catch. Neither are defensible points IMHO, but some like to make the distinction. It really is just a matter of physics, but many would rather reduce it to subjective terms to justify their own preferences. This is one of those really odd sort of debates where reason rarely ever wins...