As a rule, you don't. However, Firefox is overall more feature complete, standards compliant and willing to correct bugs in an open fashion than Microsoft is with Internet Explorer.
The ROI isn't really on the end user, it's on those who develop apps and must maintain them.
Incidentally, for many web apps there are indeed minimal system requirements: must do CSS correctly, must follow DOM event model correctly, must render pages in a standards compliant manner, etc.
Assuming consumers lead to your revenue, you should be serving them, not telling them they're not supported. But I realize that's an unpopular opinion here.
The whole "standards compliant" is now just something Web devs tell each other. Browser vendor prefix usage is at an all-time high. Devs are routinely targeting features added by browsers from draft specifications (because that never turns out to be bad). And "standards compliant" seems to always mean the latest standard, which naturally precludes any device that predates that portion of the standard. CSS 1, 2, and 2.1 are all standards too.
Oh, and my question about the consumer is given the choice between having no access at all or geolocation and drop shadows, I'm guessing almost all would rather just be able to access the site. I can't see much coming out of all this browser innovation that normal people care about or would ever ask for.
Actually, it's usually the case that the customer is forced into using an ancient and unsupported environment which is forced on them, by the custom application. That's why organisations stay with old browsers, not the other way around.
The ROI isn't really on the end user, it's on those who develop apps and must maintain them.
Incidentally, for many web apps there are indeed minimal system requirements: must do CSS correctly, must follow DOM event model correctly, must render pages in a standards compliant manner, etc.