Obviously too little contrast is bad and makes text hard to read, but it doesn't at all follow that MORE CONTRAST is always better. In fact it's pretty clear that too much contrast (which is equivalent to too much brightness, since there's a floor on how black the dark parts can get) can be a negative thing, that's why we don't all leave our displays at maximal brightness in all lighting conditions.

Another commenter mentioned that all text should just be black on white and people can dim their screens as necessary. It's somewhat a matter of personal preference, but this seems like a bad solution to me. Most people are not looking at exclusively text when browsing the web, there are lots of pictures and videos as well. Users shouldn't have to be constantly fiddling with screen brightness. In photos and videos, contrast is often used for dramatic effect, where you want some parts to get brighter than others. If the text is too high contrast so you have to dim your screen further to view it comfortably, then the photos or videos you see alongside the text won't be as impactful and enjoyable. Why should every piece of text you read be as dramatic as the most intense rays of sunlight or something in a photo or movie?