Doesn't making good use of stocks count as paint.NET ability? Using a stock isn't at all about pasting it in your work and leaving it as it is; using a stock is not necessarily the easy way to go about something, sometimes it's the way to make something new and exciting. And in an abstract piece, chances are a stock will not be the focus point. If it is, and the voters believe the work that has been done with this stock is minimal, unoriginal, or does not fulfill whatever criteria they use to judge entries,then they don't have to vote for it...
And like I said, I don't use stocks in abstracts, and I very rarely use stocks outside of that. I just think you can do remarkable things with them, that not everyone is able to do them with only superficial knowledge of the way paint.NET works, and that those who can outperform others in that field should be rewarded.
But of course, in the end, it's up to you to do what you think is right.