Registration behavior when action attribute is missing

>From the description of registration markup (3.5.1 in [1]):
[[
action of type string, readonly
An opaque string indicating the behavior class the service supports. It is
recommended that users use a fully qualified URI. If the action attribute is
missing, the intent Service is assumed to handle display of the provided
type(s) in the type attributes.
]]

What does "assumed to handle display of the provided type(s) in the type
attributes" mean in practice? 

Also, 3.5 states that
[[
The algorithm for matching intents is that the action string provided in
invocation and registration must match exactly, and the type strings must
match exactly except for possible [RFC2046] MIME subtype wildcards. 
]]
Given that the action string in invocation can never be empty/missing , the
UA will never attempt to dispatch an intent to a service page with a missing
action attribute. Wouldn't it be simpler to disallow missing action
attribute in the registration markup in the first place (except in the case
of unregistering the intent)?

- Cathy.


[1] http://dvcs.w3.org/hg/web-intents/raw-file/tip/spec/Overview.html