That helps quite a bit because it reduces the outer-bound of how much data can store with us from typical Mac/PC devices. Granted we do have people with 80+TB backed up with that service, but that VAST majority store A LOT less. The average is low enough that we can continue to provide the unlimited service - kind of like a buffet model. Some will eat more, most will eat less and buy a drink (Extended Version History).
Keeping it to Mac/PC only does make Linux, NAS, Server folks unhappy, but thus far no one has been able to crack the code on an unlimited, fixed-rate service that can back up those devices economically, but if we figure it out we'll let folks know :D