To try and put it another way, imagine, for a moment, that you are incapable of perceiving edges. Every single one of your flat tabs now looks like the same part of the same blob.
Now, realise that for a pretty high number of low-vision people, that is reality. The slight blur we experience makes most edges disappear.
(WinXP's high contrast overcame this by tripling the size of all edges).
Wouldn't the inactive flat tabs all still end up looking like one continuous blob either way? The gradient on the inactive tabs in the image I linked is a top-to-bottom gradient; it does nothing to enhance the visibility of the edges between contiguous inactive tabs.
To try and put it another way, imagine, for a moment, that you are incapable of perceiving edges. Every single one of your flat tabs now looks like the same part of the same blob.
Now, realise that for a pretty high number of low-vision people, that is reality. The slight blur we experience makes most edges disappear.
(WinXP's high contrast overcame this by tripling the size of all edges).