Basicly you need resources for something else. But, you don't want to end a task that is currently in progress.
Why not pause that task? When paused it could use something like 50-75% less resources and remain at whatever time it was at when paused until resumed.
I'm not too keen on this idea. I can't exactly put my finger on why, but it just seems like a loophole could be too easily made from it, or it won't promote newer users to go out and do missions (if they can manage with the hardware they have)