Tamsin,
We are updating our plants with additional classes so we have had to go into each plants' graphics to place parts of the plant into separate classes. We have used the Plant Objects.vwx file which ships with 2008 as a model.
When creating new plants we usually duplicate an existing one then edit the graphic to make it unique.
Many times plants are created in our office without the correct line weight or sometimes the line weight needs to be adjusted if you are working at 30 scale instead of 1/8 so we edit the graphic for these adjustments as well.
I do agree that once your plants are setup, if done correctly and how you want it from the start, then there is not as much need to edit them. For beginners or users who do not already have plant symbols there is a lot of trial and error editing of the 2d graphic before the final product is achieved.