IE10 does have a terrific default engine - it exchanges blows with Firefox and Chrome on most benchmarks. Moreover, for the first time ever, it is consistent on all platforms it is implemented on: x86-32, x86-64, and ARM.
However...
It still carries around IE5, IE7, IE8 and IE9's engines. Which can be triggered at the drop of a hat - go on and explain to a customer "you have the very latest IE10 installed, congratulations! But our website crawled to a stop on your machine because, well, tough luck, it fell back to IE7 compat mode". I had to deliver that line no later than yesterday. I let you imagine the face the customer pulled. And no, I can't send the compatibility header because, among all the layers of proxies and caches we use, it gets lost.
Moreover, there are still quite a few people who have Windows XP - and who are, thus, limited to IE8. Go on and explain to them that they have to change their PC (on which they may store their accounting and several other productivity tools, successfully and without any problem) because their OS doesn't support the latest version of their browser...
So, instead of trying to educate our customers about a matter that they don't care about at ALL, we took all of the above, rolled it up in a ball and threw it away.
We're not even going to validate our website against IE10 - we validate against Firefox and Chrome, we ensure that it runs more or less usably on a IE7 Compat, no frills installed IE, and that it issues no W3C warnings. If IE10 can load it, great! If it runs well, even better! If it goes boom, we'll just keep saying "just use Chrome, or Firefox if you already have it".