Here's one reason not to be quick to resign a game:
Your score estimation may be notably off.
In the game illustrated above, I assumed that I had lost badly when I looked at White's large areas to the top, the lower left, and to the right. White also had a nice dragon snaking through my positions. My opponent was one rank ahead of me so there was a 0.5 komi.
As the game progressed, I thought that I might as well resign. It even began to feel painful when White started some desperate territorial-reduction attempts.
As it turned out, after the scores were tallied, I was 9.5 moku ahead.
My understanding is that professionals, and some higher-ranking players, will often estimate the score several times during a game. After the middle game begins, they will keep track on a point-by-point basis.
That's a skill for me to develop at a much later date.
For those who are already interested in this subject, Kiseido has a book by Cho Chikun which is entitled Positional Judgement: High-Speed Game Analysis. In it, he describes his methods for quickly calculating real and potential territory.