That sort of treatment can render a file completely useless. Like many good things in life, too much is not a good thing. Text and JPEGs aren't good friends. By nature, JPEG tries to break an image up into little blocks and average all of the colors out. You don't want that with text.
Text has to be sharp and crisp to stay legible.
Saving a line based image as a JPEG makes it unattractive at best. At the worst it becomes unreadable. PNG supports any number of colors, alpha channels, lossless compression, and interlacing. The only thing GIF supports that PNG doesn't is animation. This file type works best with about any type of image you care to save.