Ok, let's look at a few things, shall we?
For close to eight years now, Google has been openly proclaimed as "The One Solution" and "Welcomed Overlord" by, well, damn near everyone with a keyboard and net connection. i'll admit that the do two things exceptionally well. They've got very good search results for recent materials (things become less good the older the source material), and as a byproduct, they've got a fairly good mechanism of matching ads to content. Neither of which are stellar as far as i'm concerned, and i'd really wonder just how popular they'd be on websites if they didn't pay for placement as much as they do, but that's also beside the point.
Kind of beside the point.
Truth be told, that's actually one of the bigger points. See, what google has is a tremendous distribution network with lots of little sites that run google ads on them and rake in a few cents per click to the site owner. That means that google has a huge inventory of pages they can run ads on, which means that your ads get distributed to a hideous number of pages, meaning that if your and advertiser and want the largest distribution network for your ads, you're going to go to Google (Oh yeah, and since that network is so freaking huge, better buy A LOT of clicks so that your ad stays up for longer than five minutes.) Granted a large number of those are spammy sites, but inventory is inventory.
Now, on the opposite side is Yahoo, a company that is no longer the newest, so it has a few notches against it, a company that has a bad habit of killing off the things it loves and being a bit more scattershot about what it decides it wants to be when it grows up (A portal! No, wait! A movie studio! No, wait! a search engine! No, wait! A social site! No, wait! AOL! No, wait!…) Say what you will about it, but it's a site that has a horrific amount of traffic and users and because of that schizophrenia, has something for damn near everyone (Although i'm a bit disappointed that they removed the pet horoscopes and curling sections). They also have an ad system that they use, (actually, several) and they do match a number of times. i'm not saying they're perfect, actually, far from it. Yahoo has made some absolutely insane decisions and has done more to seemingly fight against themselves than help themselves, but i'll also grant you that they're a profitable company.
The only problem is that since they're not the latest and greatest, nor do they pay people to run their ads, searchboxes and what-not, they don't have the sort of deep reach and choir of faithful that Google has. So when some of those self same choir leaders are now cautioning against Yahoo taking Google's money for ads, well, to quote a future philosopher "The avalanche has already started. It's too late for the pebbles to vote."
Now, if i were for some reason were able to hold sway over at Yahoo, i'd encourage them to do pretty much the same thing google did when they were providing search results on Yahoo's dime. Use the time, money and metrics of running google ads to build and test your own replacement ad system. Forget "building the airplane while it's in the air". That's a retarded way to build airplanes. Instead, realize you need a new one, ground the f–ker and build a damn site better airplane than the ones you're borrowing to keep your airline running. This is a chance to catch up by going completely stealth, take it. Make google have to go through the same thing in a year or two. If you were really smart, you'd take the time to build a componentized system where you can hot-swap elements so that next time some cool new thing comes around, you're not caught off guard.
Do i care that Yahoo+Google would be a huge company controlling damn near every bit of ad space on the net? Nope. It's temporary at worst and best. No matter how crappy the ad system, someone will buy space on it. Marketers want competing services because that way they can game them against themselves. If they don't have one, they'll create one.
