"If standards are the same for everybody, where's the value add for an IE or a FF, etc.?"
It's a false choice.
The standards only have to do with how a page is rendered. If all browsers had to do was render pages, the question would be a valid one. But it's a false argument - there are all kinds of things that a browser does Other Than render pages.
From a presentational developer's standpoint, I don't care if default line height is 1.2 in one browser and 1.0 in another, because if it's a problem, I'll set line height to the explicit number.
No, the problem isn't default line-height, the problem is that when I make an explicit style rule, that it hasn't made the same result across browsers. Padding: 10px should look like 10 pixels of padding in FF, IE6, IE7, IE8, Opera, Safari, etc., not 10px in FF and Safari and 20 in the IEs.
Also, pick one, and force everybody to switch. No reason on Earth I can think of that IE7 wasn't a critical update at release. What you do when you don't make people switch, is to allow people to be contrarian about the fact that they don't have to switch, and then they just don't. That may allow one set of developers to preserve code base, but it kills everybody else.
That results in more work for the presentation layer people, which is why those people don't take you seriously. And that's sad, because the more your latest browser gains acceptance and market share, the more presentation people can be confident that the code they write is cross-browser compatible.
Win, win.