The rationale for this deal has been a conglomeration of both companies' audience which would mean more eyeballs for more ads for more dollars. Nothing has been said about how the branding would work, or how the products would work amongst one another, or what the user experience would be like. Because it would probably be a nightmare. Yahoo and Microsoft have enough problems getting their own products to make cohesive sense I can't imagine what it would be like to combine them.
There was some mention over company culture clash. Silicon Valley companies HATE Microsoft. It's not just a culture clash, it's an all out hatred/oil and water situation.
I hate to say it because I loved working there but for Yahoo, this was a long time in coming. You could see the decline begin 5 years ago, and it's caught up to them. So to turn 5 years of lethargic momentum around in 6 months - don't think so.
And the kicker is, even if the merger worked they'd still be chasing Google, they'd still be very far behind. Google is wicked shrewd and continues to run with the ball.
In the short term, wall street might like the accumulation of eyeballs and reward the stock price(s) but in the long run there's going to be severe confusion.