<br /><br /><br /> Because Spanning Sync is very upfront about the data going through their servers. That's why it is billed as a subscription, not a simple one-time charge like BusySync. For myself, I'd prefer not to have my data going through someone else's servers between my computer and Google.
Spanning Sync is very cool, but what they don't mention up front is that all your data has to traverse their servers to accomplish the syncing. If you're fine with that, great, but if it gives you pause you might want to think again. There's no real technical reason they need to see your data, ever.