Sub-ID tracking would be definitely nice :)
I already started a campaign for my own product, so I can manage.
But, as soon as I get affiliates on board they would expect a sub-id tracking functionality...
Every tracking program on the market today sends a sub-id, and no one relies on pixels alone anymore, some not even bothering to ask to place a pixel.
On the other hand, as mentioned above, most affiliate scripts allow for this, so it's a huge no-no for an affiliate to find that it cannot be done.