you can have as many pk3 files as you want. With the server running xv_pure you will only use the ones the server has.
So if you server admin has some skins and other minor mods you will be able to use them.
If you go to a server without sv-pure on then all your pk3 files will be used.
I just hope this is true and we do actualy see sv_pure in the EXP
Check out
http://www.mohadmin.com on the front page I have the deffinition from a quake site about sv_pure