What's the deal with using a cheap, non-apple bluetooth mouse?


play thing
I usually use a Wacom Intuos pad with my MacBook Pro, but want to buy a cheap Bluetooth mouse with scroll wheel for browsing and such. I'm loathed to drop $70 on a Apple Magic Mouse.

Many of the mice (mouses?) I've seen come with a Bluetooth USB dongle.... this confuses me. Is it necessary, and if so why? I only have 2 USB ports so don't want to surrender one for basic mouse connectivity. Is there a cheap Bluetooth mouse that will simply connect to the MacBook's internal bluetooth?