A couple reasons
- It's cross-platform (Windows, x86 Linux, OS X)
- It allows you to make calls to POTS phones
Other than that, it's a souped up iChat AV.
Tom Sinclair
"This is a lovely party," said the Bursar to a chair, "I wish I was here."
-- The Bursar is a man under a *lot* of stress
(Terry Pratchett, Lords and Ladies)