Possibly for weight reasons? I'm in my early to mid 30s with decent near-distance eyesight, but adding my Dell 12.5" ultrabook to other things I routinely need to carry is increasingly starting to hurt. My next laptop bag will probably be a backpack to mitigate that, but it still doesn't make weight irrelevant.

Anyway, one doesn't have to go to that extent to make the point: as per the sibling reply from d_, even on a 17 inch monitor one can have issues with 120-character lines if viewing two files side by side (or 50% browser 50% editor).