I care about other ppl's experience throughout life. So, even if it will affect ~3% of the population, please do this. I think the Alt attr in html is good only for 2 reasons: a) long-thoughtful (something to say in adjunct to the picture (like a museum curator) & b) ppl who have terrible contrast in their eyes (so, if there is txt or hard to discern img). Both requirements nearly call for the same thing: a good narrative. This creative also gets pass the reluctance of developers to do is as "unnecessary" redundancy (which creates a maintenance/creative hassle). If we are "more" creative than not, it puts the brain to be able to do it (since it's for higher intellectual goals or empathic causes).
I don't believe that it's good for having something there if the img is missing excuse. It depends on the browser, firefox won't show a missing img so it may be better just to leave it alone. For ie, it's a box of where the img will be, but having non-descript txt there doesn't help. It does the user no good to know that "Limited Time: Hurry" imagery is supposed to be there (I can see it from the plain html txt).
And if the excuse is some browsers are only txt, then a txt-only version should be made available. And if are a user who doesn't want to download imgs, I can't help u; please upgrade to broadband (or it's the developers fault for a weightly img if broadband not avail). If u are a writer and don't want to download imgs, imagine reading a novel with inter-spaced imgs with non-descriptive img boxes floating around everywhere. It's not possibly. They probably like websites with mostly decorative txt and an artistic picture of some idyllic scenary on the periphery so then it comes back to a creative narrative to the pic.
Do alt for the visually-impaired and to be creative! Don't do it for the bureaucracy.