Photographers (some) were altering pictures long before computers came along. Multiple exposures, sandwiched negatives, dodging and burning in the darkroom, and of course, cropping, were all tools used throughout photography's history to create images that really didn't exist in real life.
It's kind of funny. Painters etc. have, right from the get-go, looked at scenes and then had free artist's license to paint their impressions of those scenes. Is there a tree where it shouldn't be? That's ok, I don't have to paint that. That woodsie scene would look better with a 12 point buck positioned just right and in the right pose. That's ok, I'll paint one in.
But, when photographers did/do the same thing whether with one of the techniques above or now, on the computer, they're accused of being cheats and frauds (by some). There are still some artists that refuse to accept photography as an art form. In many ways, altering the image, whether in the darkroom or on the computer, is what, more than anything else, makes photography an art form. After all, the photographer is now creating something from his "mind's eye" rather than "simply" recording something that already exists.
Oh well... that said, I do mainly use the computer to try, as much as possible, to get the colors and contrast back to what I actually saw when I took the picture rather than what the film and/or scanner actually registered. Only time I did a cut and paste was when we took a group shot and didn't have an extra person or tripod to take the picture -- had 2 people take the shot separately and then dubbed one of the photographers from one picture into the other. Worked pretty well too! Doesn't mean I won't consider doing something more drastic in the future if the creative urge strikes me but, for the most part, I like to record reality rather than fantasy.