I feel Ajinca would be a good rotation big. Maybe not an ideal starter, but might be an upgrade over Zeller defensively. Remember that the Bulls did very well by rotating 3 rather mediocre centers. They key is that they don't hurt you while they are out there.
Then again Bulls had transcendent talent in Jordan, Pippen, + 3rd star (Kukoc, Grant, etc.)
As long as he doesn't cost a lot, I'm in favor of adding Ajinca. Perhaps our future starter at the 5 will arrive through the draft, but most likely he won't be ready to start as a rookie.