Actually, right now there is a ton of competition... from non-unified sources. SIRIUS and XM are both working as a broadcast provider that is non-FCC regulated. When they first launched birds up, this seemed like a kind of niche market. But now, there are companies putting out MP3 ready head units for cars, etc. and so SIRIUS / XM are now in direct competition with: PodCasts, Internet Radio, IPod, and now, with streaming online content to it things like the iPhone are really changing the game.
In the reality of it, they aren't competing with each other as much as they are one niche product competing in a large field of niche products as content providers. Preventing them from merging only pretty much guarantees they stay relatively small and never really act as a true competitor to any; they have nowhere near the market penetration, even combined, of newer cell phones which can receive (and play) internet radio ... the drive to say "Merging is always bad" sometimes is defeatist to the consumer when a failure to merge just means the end loss of both companies or the death of one or the other, and so consumers have fewer choices in the end then they would if a merger were successful and provided a solvent company
