Finally Yahoo has come up with a better web-based messenger. This should have been done long time back when Google started integrating gtalk on the web in every means it would - Gtalk is intergrated with gmail, google personalized home page as a gadget and finally you can add their gadget to any page that you might want.
Of course right now gtalk is not used for any kind of revenue generation. Whereas yahoo web messenger screen shows Ads once you've logged in.
All said and done, you have one more PC-based application available in a web-based form. The whole world seems to be moving towards web-based applications!! :)