UDDI Subscription API Optional?

I noticed in UDDI v3.0.2 [1] (and in 3.0.1 and 3.0) that the Subscription 
API is OPTIONAL, and the replication API is recommended (SHOULD offer) 
(with a caveat related to multi-node registries).

Within the DoD (my customer), there are a number of organizations that are 
considering using UDDI, and I expect there will be several independent 
registries.

One approach to federation is to subscribe to each of these registries so I 
can be notified of changes that interest me.

I would have thought that the subscription API would have been at least 
recommended (SHOULD), and possibly mandatory (MUST).

What was the rationale for making it optional?

There does not seem to be many implementations of the Replication API; 
perhaps MAY is more appropriate than SHOULD.

[1]  http://uddi.org/pubs/uddi-v3.0.2-20041019.htm#_Toc85908033

Paul