Here are other considerations.
When text is an image and you zoom in or out in the browser window, the image will not scale and/or renders as an image and will be pixilated and look softened when zoomed. Text is still sharp at 1600%.
Try it in Safari. Zoom with or without text-only.
Also, when you select text you can use the Speech function in OS X to read the text aloud.
Do search engines give proper text more importance than image text, or anything like that?
I read that Google uses some 500 variables to index a page and rank it.