I fully agree. I almost never play the whole game against computer. But playing from specific position is a wonderful way of training.
I take positions from:
- my own games
- master games
- endgame books
- tactical excersises (because winning a minor piece due to combination is one thing, but being able to finalize it against 3000+ beast is something else)
It's also nice that unlike live chess you can interrupt it at any moment and continue later.
When I win by resignation, I have begun finishing the game vs computer. As a novice player, I learn a lot by this process. Often the computer wins. And almost always, the computer finds a play I did not see. I have learned that seemingly hopeless positions are often not as hopeless as they seem to me and vice versa. Perhaps stronger players would not find this as useful, but at my level, it is very educational. Generally, I do not like playing against computers since they do not play like humans and I do not find it enjoyable. However, when a computer plays me from a position that my opponent thought was finished and finds a play, I learn a lot.