SiriusXM benefits from more people listening to their service. Initial cost of the radio may be holding people away from the service - especially the high cost portable units. Mine still retails at $199, and I bought it three years ago. I got it on Amazon for $119ish, but that's not the point. But then on top of that I had to add a car kit for $50. I'm pretty sure they include that now...
Having an app out there will allow SiriusXM to reach more people. People love the app store and iPhones. Any watercooler will have buzz about apps now and then. That and XM was developing their own iPhone app half a year ago that nobody has seen since, but their product teases are notoriously long. Look at the XMP3 player, that was pushed as the new hotness for at least fifteen months before it came out. Then there is an official XM app for BlackBerry and some other select phones, even though it sucks, so the company has at least shown interest in mobile web delivered radio before.
Easier accessibility to satellite radio can only increase subscriptions. In addition to the cost of the radio, as mentioned before, that radio has to see the sky. I can't use my Inno at the gym worth a damn, but I can use my BlackBerry there no problem. Inside your house too, if your radio isn't docked with an outdoor antenna attached, you're screwed. (Unless you have the Howard radio with wifi... which is $329 plus accessories... and maybe you have wifi at the gym... but my sub is XM so I don't have that option, blech)
A large portion of satellite radio subscriptions are car-based, and a lot of those cancel after their free three months to one year that was comped with the vehicle purchase. SiriusXM would love for you to take that radio you pay for, and keep paying for it, listen at home and on the way to work, then wherever you go online. It would make Mel Karmazin's day to have this app. It could save satellite radio.