Many GUID generators pick markers from the generating machine that´ll increase the uniqueness of the random seed. The machines MAC address + systen nano time is considered a minimum to achieve a quality seed.
It's uncommon for MAC address to be used. To start with it's not guaranteed to be unique, because some companies re-use the MAC addresses to save on license costs. Also you don't want to give out your MAC address. Lastly with your MAC address taking up so much of the space, it becomes much easier to guess what the next number will be.
A good guid generator won't create a predictable sequence
http://en.wikipedia.org/wiki/Globally_Unique_Identifierhttp://en.wikipedia.org/wiki/Uuid.
You really want to be using v5, although Microsoft don't.
