In games, it just means my eyes have more area to scan. And for reading and writing text, or even editing images, I naturally tend to size my windows into a 4:3 area anyway, leaving dead real estate "bars" along the sides.
For example, here I am in Windows, working on my "Star Control: Origins" review. Observe my "working" aspect ratio:
Another disadvantage comes when emulating older games, such as with console emulators, or DOSBox: I hate playing 4:3 games with 16:9 dead space on the screen.
I notice that a lot of commercial collections will "fill" that space with artwork, which makes things even worse, because the static art is disorienting when put next to moving pixels. There just is no good solution, which is one of several reasons why I play older games on a 4:3 CRT.