An Internet Draft as reference material

Dear all

What do you think about "de facto" that many technical documents
are currently using Internet Drafts as referece material?

I've seen next two cases:
1. An Internet Draft refers to another Internet Draft.
2. A book refers to another Internet Draft.

I'm not sure of the next case. Any body observed this?
3. An RFC refers to an Internet Draft.

Hereby I'd like to ask your opinion, esp. on no.1
Is it legitimate?

Cheers,

Jiwoong Lee
KT M.com