I have a Humbucker - Singlecoil - Humbucker in my Yamaha, and 5 selector settings.
I honestly have no idea if there is a particular use for which one or if there is one you "should" use during certain circumstances.

Mostly i keep it on the one near the bridge mostly because it sounds nicer in general. I dont think ive ever used one of the two "inbetween" settings
depends what you like and what sounds best really. Some people prefer the neck pickup for soloing, some prefer the bridge, its up to you really. Single coils are really nice for sparkling cleans tho
My Gear
Line 6 Variax 600
Epiphone G-310
Ibanez RG350EX
81' Ibanez Blazer Bass

Line 6 Spider III 75
Line 6 POD X3 Live
Fender ToneMaster Mini
Boss DD-2 Delay
Boss CE-2 Chorus

My Band
Band MySpace!
Band UG Profile
heres how id do it... all bridge humbucker for my metal riffs/rhythems single coil for my cleans (and some soling) neck for arp solos and the neoclassical solos and back to bridge for thoose sick pentatonic/bluesy metal leads
The bridge humbucker is best for distorted rhythm parts and straight up soloing, it's bright, fat and probably has quite a tight sound. The singles will have a "looser", airier sound and are good for less distorted parts - the neck pickup on strat-type guitars is very expressive and good for soulful lead parts. The in-between positions (2 & 4) are great for funky-sounding clean tones, especially if you throw a bit of chorus in there. They're also good for clean tones in general, whist with a morew overdriven sound you'll get great hendrix and SRV type tones.

I don't think I ever use my middle pickup on it's own...I should really learn to
Actually called Mark!

Quote by TNfootballfan62
People with a duck for their avatar always give good advice.

...it's a seagull

Quote by Dave_Mc
i wanna see a clip of a recto buying some groceries.

Quote by e-bowie
Mostly i keep it on the one near the bridge mostly because it sounds nicer in general.

There you have the answer to your question - they sound different.

Experiment and see if you can get tones you like out of the different pickups, and if you don't like the tone you can get out of them, then obviously you just do not have to use them (or consider upgrading to ones you would like).

Personally I have mine set up so that I can get either a distorted tone, or a clean-ish tone, just from which pickup I use (two humbuckers which have separate volume controls).