Luthiers use a scarf joint when building a guitar neck out of a single piece of straight wood about the thickness of the neck. The wood is cut at a slight angle and then flipped to provide (a) an "angled-grain" joint which is stronger/ less likely to snap under stress or impact, and (b) proper tilt to the headstock which creates greater string pull for intonation.
Along with strength and stability, another advantage is economics. 1" Thick pieces of maple are cheaper than 3" thick (and easier to create from quarter-sawn boards), and you would need the bigger one in order to cut a headstock with an angle like you would find on a typical scarf-jointed headstock. Plus, when you cut from the bigger block, there's a lot of scrap wood left over that isn't really big enough to do anything with. Well, a good doorstop maybe...
Necks cut from single larger blanks typically have a volute (hump) at the nut to provide the extra strength needed. Additionally, volutes look pretty cool- so cool that many neck-makers create a volute on their scarf-joint necks to give them that antique look (ala Martin, etc.).
Jems nowadays have scarf-joints (I think- I'm very open to correction here) and examples can be seen at
http://www.jemsite.com/jem/necks.htm.
Hope that helps!
