How much have they lost in sales from not having these features? Not much. How much could they gain by improving them? Very little.
How much could they gain by becoming bigger in the various businesses Yahoo has (IM, webmail, web portal, search, photo sharing, etc.)? How much could they gain from synergies between Yahoo products and their own (e.g. tying Flickr into whatever photo management software comes with Windows)?
There is probably more value in even minor bits of the Yahoo deal (closing down Zimbra in favour of Exchange server, for example), than there is in improving the Windows desktop.