No need for a leader if you're running light line--8lb test and down, or higher in low-visibility conditions. I've had luck drifting by sliding a piece of tubing up the line into which I insert the lead, then using a turle knot to attach a hook (forms an egg loop). Or use split shot.

Used to rely on this rigging when fishing a terminal area with hordes of people. The quicker you can get your line back in the water the better; if you lose it all you have to tie three knots using the snap-swivel/leader/hook rigging unless you've got leaders pre-tied. Naw--just slide a corky on your line, tie a hook on, and pinch some shot and you're ready to go. Downside is that you'll run through a spool of line quicker--but no big deal if, like me, you swap it out for a new spool each month or so anyway.

Some situations demand leader material, however--in low, clear water, use a long light leader...yes fish will see your 20# test Ande, and yes they will avoid it. Leader is nice for applications involving some sort of spinning thing--or, might as well use leader, since you'll be using a snap or barrel swivel.

turle knot: http://www.fishandfly.co.uk/knots/turle/
http://shoreangling4u.tripod.com/knotguide/id24.html
http://shoreangling4u.tripod.com/knotguide/id24.html

Having said all this I must admit I always use leader material at least 2 lbs lower than my mainline, and usually have at least a barrel swivel joining them...so really I've found the answer to "Why use leader?" is "Because it works."