Do people put these background patterns behind text expressly to hamper legibility?
Because a pattern like that looks quite alright with a contemporary 800x600 resolution 15" CRT monitor, probably a lot more pleasant than black text on plain white background.
Nope, I was bitching about those *I#$@! backgrounds way back then. It was part fashion and part narcissism: "Look ma, what I can do. I'm a real programmer!"