Another advantage of doing projects is that we tend to get better with time. Not only was Pepper # 30 Edward Weston's thirtieth attempt at a pepper, he'd also been working with a variety of other vegetables and shells. Although his other vegetable images do get included in shows and books, it is the single wonderful image that many consider one of the greatest photographs ever. So it would appear that even though this was part of a project, it's ok to come out the other end with a single iconic image.
Given a choice between being known for 20 mediocre images vs. one great one, I think most of us would vote for the one great one, in the hope that having made one, it wasn't a fluke and we can do it again, eventually.