Any good trader will never use AON. It practically ensures a longer than normal fill.
If one is using a broker that routinely gives you partial fills it's time to change brokers.
Good brokers use execution speed and accuracy as selling points in the highly competitive market of which one to choose. The old adage "you get what you par for" is applicable. One of the drawbacks of using deep discount brokers would be having to deal with problems like bad fills.
I use Scottrade and almost never get partial fills. If one occurs they are the first to admit that it was a bad fill. Unfortunately, all you usually get for the inconvenience is a few free trades. But, like I said, I almost never get partial fills and never use AON.
Hope that is of some help.