If you are never going to use the raw files, you might as well shoot jpg only. However, if you normally like shooting raw and just need a quick jpg now and then to email or upload, go for it. The camera DOES NOT destroy the raw file, it just generates a jpg and leaves the raw file alone. I don't think you can do a batch, only one conversion at a time.
I find the function confusing though because it gives the jpg a new file sequence number. For example if you have a 1234.nef file you get a 1235.jpg (or whatever the next available sequence number is). If you have several very similar shots on the card, it would be hard to identify which nef file was used to produce the jpg.