I do think that sites should strive to "just work" with nice system fonts (and respect user agent settings), but this is just an obtuse answer.
You can ship your own fonts. Everyone and their dog uses one of two web browser engines. People want their websites to "look right". You can offer a fully specified look!
And there is always someone on XP who complains your website looks like crap because their defaults look like crap while every other website bundled it’s own fonts and inputs.
You can ship your own fonts. Everyone and their dog uses one of two web browser engines. People want their websites to "look right". You can offer a fully specified look!