I also wonder about this, i did attempts of creating ply files but they just weren't working when i tried to import them.
Sry for this answer, it doesnt really help, i just hope that i get emails from now on when someone reply in this topic, that's why.

Out of curiosity, as I didn't know this format until now, what are the advantages of this format over other 3D formats supported by Processing libraries?

PLY is the standard format for saving Points cloud information. i.e. The location of several points. This is then used in a program for example Meshlab to then over layer a mesh on top of this data to create a 3d object. The x,y,z of each location is taken either from a laser line of a depth field image where nearer colours are lighter then distance colours. This is easy to achieve in processing its just the exporting to the PLY file format, that I am having trouble with. How ever Ideal 3ds would be even better.

I saved text file in following format:

ply
format ascii 1.0
element vertex ##
property float x
property float y
property float z
end_header
# # #

where : ## going to be number of vertices

example file:

ply
format ascii 1.0
element vertex 4
property float x
property float y
property float z
end_header
-355.98538 393.78912 1814.0
-352.83502 393.78912 1814.0
-363.42184 388.48538 1804.0
-360.28885 388.48538 1804.0


worked well in meshlab.
look also superCAD library. it has very simple obj file exporter and it is way faster than writing large string to file.