I think the issue is mostly what kind of fishing you are doing, what kind of main line you use, and
what species/size of fish you are after. If you use any mainline other than the transparent polymer type
such as mono or flurocarbon then I suspect you don't have much choice but to use a leader. I troll alot for trout using leadcore which is essentially a braided line. A leader is a must because the mainline is highly visible. Furthermore it's really remarkable how much wear and tear goes on the leader during the course of a season from abrasion and fighting fish. I suspect that the flippin' Bass guys who have to horse strong fish out of cover use leaders on the braided mainlines perhaps not just because of the visibility and wear and tear, but to provide some stretch as well since braided doesn't. In terms of species if we had muskies and pike in the reservoirs we'd all using tough leaders, in some cases like bait fishing maybe even wire leaders. In theory the leader is suppose to be stronger than your main line since it's suseptible to more wear and tear and takes the brunt of the stress. I have in the past run into trout fishing old timers who use a weaker leader, on the grounds that it's less visible to the fish. Maybe so, but with the advent of flurocarbon which is "invisible" to fish that type of thinking no longer makes sense.
Anyway, unless you're using a non-transparent mainline, I don't think it's absoutely necessary, although
it's probably a good idea.
