I use Spotify on my Android phone (Samsung i5700). Even with the tracks synced to my SD card, the music can stutter if you're trying to browse the web at the same time as listening to music. Streaming over wifi and browsing is completely useless.
I'm assuming that there are overheads inherent in the data reception and processing that cause this, and thus dual-core makes sense. Quad-core? I guess it's the same issue as found with full-fat PC CPUs - is Quad-core that helpful unless you have programs that can utilise the extra available threads?
Another factor could be energy-efficiency - is it more efficient to have extra cores that can switch themselves off when not required, or have fewer cores that are sometimes overwhelmed? There's marketing in there too of course: "Hey, my phone's got more cores than your laptop...!".