Are there any particular advantages to using the viewfinder for focusing instead of the LCD?
In general there are the following advantages of manual focussing via the optical viewfinder instead of the LCD:
The viewfinder image is almost certainly sharper than the LCD, when viewing the entire image. This makes it easier to judge when something is in focus for the times when you need to be able to see the whole image at once (e.g. for a scene that may change without warning)
The viewfinder image updates in realtime whereas the LCD has a small lag due to the need to read and process the sensor image, as well as the finite refresh rate of the LCD screen. This could have affect focusing speed as you can turn the focus ring faster using the viewfinder as you can see the changes in focus more immediately.
A viewfinder is easier to use in direct sunlight (though you can get a loupe/shades that make LCD viewing easier).
Viewfinders in SLRs have the option of using a split prism focussing screen which allows easy manual focussing on subjects with sufficient detail.
DSLRs with live view limit the amount of time you can view the image live on the LCD as it can heat up the sensor. The time is quite long, however for all day shoots in hot conditions sensor overheating may be become a disadvantage of using the LCD screen compared to the viewfinder.
Similarly the shutter is open the whole time you are using the LCD so if you inadvertently point the camera at the sun (or worse put the camera down pointing at the sun) you can damage the sensor.
There are also many advantages of using the LCD instead of the viewfinder to manually focus:
The flipside of the last point, if you point the camera at the sun using the LCD you'll only damage the camera, not your eyes!
If you don't need to monitor the whole image you can zoom the LCD image into the area of interest to see more detail than you can through the viewfinder.
The LCD image you see is always has the exact DOF you will get in the final image if you focus stopped down. Most laser etched focus screens in SLRs cut of light after about f/2.5 meaning the DOF will appear greater in the viewfinder when using fast lenses wide open.
For compact cameras with optical viewfinders there is a paralax error meaning you see a slightly different image than you capture. Again DOF may be different. The LCD shows you the correct image.
Most SLR viewfinders only cover 95% of the image. If for some reason you need to focus on the extreme edges you need to use the LCD screen.
The LCD screen is more visible in the dark due to the ability to brighten the picture by increasing ISO.
And on top of Matt Grum's almost exhaustive list, viewfinders won't eat up your batteries.
(Except if the viewfinder is electronic, but that's luckily quite rare)
And to top it all: using the viewfinder almost forces you to hold the camera in such a way that it will be the most stable platform possible without resorting to a tripod. Thus you'll suffer a lot less from motion blur, composition errors, camera shake, etc. etc.