The problem is: How do you trigger the IE7 compatibility mode? Microsoft proposed having new pages identify themselves as new with an "IE8" tag, and people freaked. The opposite answer -- having the old pages identify themselves with an "IE7" tag -- doesn't work because it requires someone to change all the old pages, and until that happens they're all broken, so the corporate users will refuse IE8 and stick with IE7 for another decade.

Having users press a button for IE7 mode whenever they see a broken page is just asking for trouble. Now you've equipped every Solitare-playing cube dweller in your company with a "break this page" toggle switch that needs to be constantly moved back and forth. The call volume to the IT department help desk just went up by an order of magnitude.

So you're stuck with stupid hacks like "enter IE7 compatibility node if the URL matches this regexp". Fragile, fragile, fragile. And what if you actually have the budget to slowly migrate your ancient intranet site into the modern world, one page at a time? How do you serve half your pages in IE7 and the other half in IE8 without a special tag to distinguish them?