If I have a photo folder named "cars" and I take a picture of a new car some day and want to add it to that folder on my iPad, is there no other way other than completely removing the folder and then re-adding the folder again with the one additional photo added to it?
You don't have to remove the entire folder to add a single photo. You place the photo you want to add in the folder of photos that has already been transferred and only the photo added to the folder is transferred when syncing. To remove a photo or photos without removing the entire folder, move the selected photo or photos out of the folder followed by a sync.
I use iPhoto on my Mac for importing photos from a digital camera including from my iPhone's Camera Roll and for photo storage and management on my Mac. I make use of iPhoto albums for select photos just as I make use of iTunes playlists for select music. I select the iPhoto albums I want transferred to my iPhone followed by a sync. To add an additional photo, I can add the photo to an existing iPhoto album that I've selected to be transferred to my iPhone followed by a sync which will transfer just the new photo. To remove select photos, I remove them from the iPhoto albums or albums in iPhoto followed by a sync which removes those photos only. To remove an entire album, I deselect the album under the Photos tab for my iPhone sync preferences followed by a sync. To add a new iPhoto album, I select the new album under the Photos tab for my iPhone sync preferences followed by a sync which transfers the new album only without any other albums or photos being touched on my iPhone.
To me that puts too much wear and tear on your device removing the content and re-adding it all of the time.
All content does not have to be removed and re-added all the time - you obviously don't know how the sync process works. Wear and tear how, and how is this any different from manually dragging individual photos?
If you don't want to transfer photos via the iTunes sync process, you purchased the wrong device. This is how it is done with all iPods, with the iPhone, and with the iPad.