Brandon Clark raised a good point in off-list e-mail and we came up with a solution that would help people creating alternate adapters for Zend_View (e.g., Zend_View_PhpTal).
Because PHPTAL templates only have access to the variables passed to them (and no reference to the Zend_View_Interface instance), calling helpers requires either:
1) Explicitly assigning $this to a template variable, which breaks encapsulation, or
2) Having Zend_View_Abstract::$_helper be a reference to a Zend_View_HelperBroker object, like the action controllers have Zend_Controller_Action_HelperBroker.
This seems like a logical split to me. Thoughts?