By creating a static-size page file, you avoid the following data layout situation arising on your hard disk (Where X is data and P is the page file):
XXXXXXXXPPPPPPPPPPPPPPPPPPPXXXXXXXXXPPPPPPPPXXXXXXXXXXXXPPPPPPPPPPPPPPP
Defragmentation theory is all about consolidating data for speed purposes, therefore having the following layout:
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXPPPPPPPPPPPPPPPPPPPPPPPPP
Theoretically, should enhance the read speed of X.
However, whether or not there are any noticable speed improvements would depend on what one was doing with the data. For example, on a fragmented drive imagine a huge movie file being played on an old slow machine, or imagine a big collection of small image files no more than 1 - 2 megabytes each; In the case of the movie-file, one may experience performance issues, but in the case of the small image-file scenario, performance issues would be negligable.
As concerns Windows stability and page file sizes;
I have also experimented with having no page file as well. Depending on how much RAM you have in the machine, performance is likely to be quite slow and "jittery". Also, some high end games may refuse to start unless a page-file is active.
As a general rule, page file size should be equal to or greater than the amount of RAM in your system.