Do they always wear the same mask? In Tokyo Ghoul, each ghoul is provided a mask to hide their identity from the CCG. And although I've yet to watch the entire series (currently on season 2), it appears that ghouls always wear the same mask. Doesn't this cause a major disadvantage during battle?

If a ghoul wears the same mask all the time, then upon first look, an opponent will know the capabilities of the ghoul, should the ghoul have been encountered before. But, if different masks were worn, and/or traded around, this would provide uncertainty when beginning a fight, and would be much more advantageous for ghouls.  

So, is there any particular reason a ghoul will wear only one mask?
From the website : http://tokyoghoul.wikia.com/wiki/Mask


  Masks (マスク, masuku) are worn by ghouls to prevent their identities as
  "humans" from being discovered by the CCG. Several masks bear a
  symbolic attachment or representation to their owner. They heavily
  influence the alias of some ghouls filed under by the CCG.


Ghouls wear those masks because they want to live alongside with the humans without the fear of getting caught by the CCG. This is the only reason for wearing the masks. They aren't afraid of being identified as specific Ghouls while fighting the CCG. Changing the mask will be like changing your very identity. It will be clever for a Ghoul to change masks from time to time but as mentioned earlier, it's difficult to change your identity (and everyone is proud of being themselves).
This seems to be the only explanation!