Thanks, yurigoul. Yup, the newletter module will be(or has already been?) decoupled from userdefinedform. We just didn't officially announced this due to the new features lack of testing. But that is what we think the right way to go.
By using the new "SubscriptionPage" you are be able to define a form with all fields available from Member, Member's subclasses, Member's decorators as the form field candidates, each field can be customised with its own label, its own validation message in CMS, etc.
The reasons to decouple the module from UserDefinedForm are, besides what yurigoul mentioned above, also because if an back-end user defined some fields that doesn't exist in Member (and its subclasses and decorators) in his UserDefinedForm, then he will lost in where these data goes and how to retrieve them.
In general, all modules for SilverStripe core should be only depended with SilverStripe core, and less related with other modules to keep them neat and work all the time. UserDefinedForm are dying, we just don't want the two modules coupled.