There was a lot of buzz last week about the XM SkyDock, Sirius XM Radio's (NASDAQ:SIRI) second attempt to turn Apple (NASDAQ:AAPL) iPhone and iPod touch owners into satellite-radio subscribers.

In the wake of the SkyDock's rollout yesterday, the device's shortcomings will probably prevent it from moving that needle. It's cool -- but not cool enough. For every attractive SkyDock feature, there's an equally compelling hype deflator.

Good: The XM SkyDock will charge your iPhone or iPod while in use. Forget the knocks claiming that listening to premium radio will suck your smartphone's battery dry. Your iPhone or iPod will pop out of the SkyDock even stronger.

Bad: The diagrams filed with the FCC looked too bulky to offer a portable solution, and now we know why. The device is limited to vehicle usage, since it feeds off cigarette lighters for power and plays through car radios. In short, it's more restrictive than the portable models already on the market.

Good: Unlike the Internet streaming app that came out two months ago, SkyDock is an actual receiver. Subscribers will have access to everything that satellite radio is beaming, including the play-by-play sports and Howard Stern broadcasts that aren't available through the App Store application that came out in June.

Bad: This is an XM device. Stern and the NFL are on Sirius. So instead of going with a Sirius receiver and paying $12.95 a month, Stern fans that get SkyDocked will need to sign up for the monthly $16.99 "Best of Sirius" package. If they want to make the service truly portable, they will then have to tack on an extra $2.99 a month to stream through their iPhones (but won't get Stern outside their cars). A portable receiver would have all that -- and Stern on the go -- for the base price of $12.95 a month.

Sure, it's cool that the iPhone can become a slick touchscreen controller for a car-based XM receiver. Unfortunately, this isn't a Palm (NASDAQ:PALM) Pre, capable of running several applications at a time. What happens if you want to run a GPS app? What happens when a call comes in?

I'm a satisfied iPhone owner. I subscribe to both Sirius and XM. Convergence -- at a premium -- is no slam dunk. Apple is a developer magnet, with thousands of apps shouting "pick me!" in the App Store. It's hard to get noticed in a crowd, especially when you're competing in a realm of fierce, no-cost rivals.

iPhone owners are paying AT&T (NYSE:T) $20 to $30 a month for unlimited data plans that give them access to free apps including Pandora, imeem, and Time Warner's (NYSE:TWX) AOL Music.

Wireless isn't the future for satellite radio. XM has offered limited programming at discounted prices for years through its XM Radio Mobile platform. It's available through Alltel, AT&T, and select models of Research In Motion's (NASDAQ:RIMM) BlackBerry.

"Sprint (NYSE:S - News) customers will be the first wireless customers to enjoy SIRIUS Satellite Radio programming via their mobile phones," reads a four-year-old Sirius press release.

In other words, mobile convergence isn't new. It hasn't moved the needle in the past, and it won't now, when the free ad-supported alternatives are everywhere.

There's nothing wrong with approaching the smartphone market in a cost-effective manner. Don't scoff at the potential for incremental subscriptions. However, throwing Apple and Sirius XM into a press-release blender won't result in a hype-worthy smoothie. 

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Longtime Fool contributor Rick Munarriz is a subscriber to both Sirius and XM. He owns shares of TiVo and is a member of the Rule Breakers analytical team, seeking out the next great growth stock early in its defiance. The Fool has a disclosure policy.