Google will build a maps app for iOS. They can't afford not to.

New rumors this afternoon suggested that Google was trying to ship an app by Christmas (credible) or had already submitted one to the App Store (less credible). Google itself has all but said it is building for iOS: "Our goal is to make Google Maps available to everyone who wants to use it, regardless of device, browser, or operating system," a spokesman told CNET in an e-mail.

Some Android fans are wondering why. Here's a key advantage Google has over Apple, they argue. Why not keep it from iOS? Heading into the crucial holiday season, you could flood the airways with advertisements pitching Android as the platform with working maps. Put up a screenshot of the melting Tacoma Narrows Bridge in iOS 6, the argument goes, and watch sales of the Galaxy Nexus shoot through the roof.

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It may be fun to think about, but the business case for it is weak.

For starters, Android had a superior maps app long before Apple's came along to showcase monster truck ramps on the freeway. Google Maps for Android boasted turn-by-turn directions, 3D city maps and offline viewing long before its iOS counterpart did. (Offline viewing never came to iOS at all). Nonetheless, the iPhone -- which is more expensive than many Android phones, and available on fewer carriers -- has maintained steady market share. (It also grabbed 77 percent of all profits in the mobile industry in the most recent quarter, but that's another story.) The point is that even a vastly superior maps app will not by itself move the market-share needle for Google.

More importantly, Google needs the traffic that iOS users bring. It helps them in at least two big ways. One, all those millions of iOS users make Google Maps better. The analytics from the iOS app are invaluable to Google: did a user make it from her original destination to the place she wanted to go? How many times did she check the map before getting there? Did she have to constantly re-center the map along the way? All of those are valuable signals to the maps team, which uses them to identify trouble spots and improve the overall experience.

And iOS users loved their Google Maps. According to ComScore data , about 45 percent of all mobile traffic to mobile versions of Google Maps came from iOS. That's 31 million users a month, spending an average of 75 minutes each inside the app. iOS users are more likely to use the app every day than Android users, and spend more time with it when they do. In April of this year, 90 percent iOS users opened Google Maps at least once -- compared to just 71 percent for Android users.

Put another way: iPhone owners tend to be Google Maps power users. The kind of people who improve the system in a way that algorithms can't. Google won't want to give up the huge traffic they bring with them.

"The only way of validating whether the algorithms are right is throw users at it," said Scott Rafer, CEO of Lumatic , which makes city mapping apps. "And if you've got users, you don't need algorithms." The second big way iOS users help Google? Revenue. Google hasn't yet begun aggressively rolling out advertising in its maps product. But local commerce is a high priority for the company. iPhone users tapping around inside Google Maps will allow Google to target special offers and other advertisements directly to users based on their location. If they can figure out an elegant way to implement those ads, Google stands to earn hundreds of millions of dollars in new revenue. The company won't pass that up just to make Android look good.

Besides, there's a third option here: build a Google Maps app for iOS, but continue to give Android all the best features first. Google already takes this approach with YouTube, where product managers refer to the Android app as the "reference" version. When product managers went to redesign the mobile app, they started with Android and developed the iOS version later. Today it's a win-win for the company: YouTube still gets to profit hugely from all the advertisements running on iOS, while reassuring Android users that they're the company's top priority.

So Google Maps will make its return to iOS -- the only question is when. In the meantime, iPhone users who miss it badly can visit the mobile Web version using their browser of choice. It works just fine, and Google is really missing your clicks.