Well, if you do a lot of cooking you'll need to grow some, for some of the recipes. You can buy rice, but you can't buy some of the others.
For just shipping... they're okay I guess? You have to plant one per square rather than one bag taking up four squares, like with regular seeds, but if you fill the paddy up you won't have to water it again for a couple days, so that's less of a pain.
The biggest pain though is that since it's one seedling per one final crop, you can't increase the quality, use them, and still have some left over for seeds. So if you want a five-star paddy crop for the crop festival or something, you basically have to just keep growing and fertilizing it up all the way, and just save it. If there is a way to get multiple five-stars from one growing, we haven't found it yet. (Though it's possible you might be able to do it with big/giant crops, but I don't know if paddy crops get those.) --- Please insert witty quote here.