I came up with this theory back in 2009 when I was reading the Kanohi section of BS01, and I was looking at the Golden Kanohi page.
Now, we all know what the Golden Kanohi are: Six golden masks with the power of the six masks worn by the Toa Mata all in one. In order for the Toa Mata to obtain these masks, they basically had to give up their six Kanohi each.
The Toa were then able to use the various powers within for a variety of different things, from facing Makuta to collecting Krana. The Toa Mata would not have had their original six masks because they “traded” them for the Golden Kanohi they then wore.
But what would had been the purpose of this trade. Sure, they wouldn't have to worry about losing the other five because the others were all in one mask instead of their Suva. This I believe is the key.
The shields of Karda Nui are strong enough to prevent the swap of a mask with another from a Toa's Suva. The Golden Kanohi would not have had need of the other masks stored at the Suva, because they basically wouldn't be able to in the first place. Without the Golden Kanohi, any danger within Karda Nui would have probably been difficult for the Toa Mata to deal with should they need any of the their other Kanohi . Without the other masks to call upon, the Toa would have had to be pretty smart in their fight or what ever circumstance there was, if there was any.
So, my theory is thus: The Golden Kanohi were meant to be a way to help arm the Toa Mata should any danger be within the core. They were basically a foresight for a possible situation there. Also, there was mentioned once of multiple ways to awaken Mata Nui, so maybe one of the ways to waken him would have required them. But it would seem that the Golden Kanohi would have had a purpose in Karda Nui.
What do you guys think?