Don't rely on Smarty singleton to pass data

I've noticed many bugs have been hidden by the fact that Smarty templates are a singleton. To illustrate, CRM_Event_BAO_Event::sendMail is supposed to get its variables from a function argument. Code in that function is testing data coming in through that argument, yet the data is not passed through to sendMessageTemplate. Instead, code like CRM/Event/Form/Registration/Register.php :: processRegistration is calling assign(), and the data is only appearing in the generated emails thanks to the singleton template object.

This is very unclear for the programmer, and also a fragile practice since the data in the singleton object would be overwritten if another message was sent (or a form rendered) after the first assign() call.