What's the point of...

It shows that he was, in fact, working with them at one point before he either messed up, grew a pair and tried to stop them, or they simply didn't require his services anymore.

Personally, I think the ending of Cube Zero just cheapens the ending of the original Cube. I really enjoy movies that allow the audience to form some conclusions of our own, to a certain extent, and the ending of Cube Zero has taken that away from me. They were aiming for the "Oooohhhhhh!!!" ending but they missed completely. It may have been justified if Cube Zero actually offered any new information to the mystery, but it didn't... so poor Kazan is has just been reduced to being a schmuck whether that was how any of us envisioned him or not. I would have rather simply not watched Cube Zero in the first place.

I've always thought that Kazan and Wynn were the same person since the dialogue between the people who find him/them in the cube is exactly the same.
Holloway goes up to Kazan in Cube and prevents him from banging his head against the wall exactly the same way as the older, black woman in Cube Zero does with Wynn. And there's the exchange about the colours of the rooms etc.
Same people, they just look different.

I'd like to think that Kazan/Wynn gets through the white room, says "Yes" (if there is still something within him that can remember what happens if you say "No") and is let out.
But one of the reasons why I liked Cube so much was because we didn't know what happened to Kazan.
Cube Zero ruins that a bit.