I'll have to say, Blizzard has given us certain functions within their API, which addon authors can use to build their addons. Any feature Blizzard has given us to use, should be considered fair game to be used.

Blizzard has in the past, changed what an addon can and cannot do, by changing the API features/abilities. The original Decursive comes to mind.

You will never be banned/suspended for using an addon, because addons can only do what Blizzard has specifically permitted.