Steven,
It is obviously a flaw solution to extend model layer just for the sake of ability of visual transitions (at least in general case). I think that 'removing' is meant to be some new event that fires when element is going to be removed but still is in DOM. So if it would be possible to catch that event and pause it so that after resume (for example when transition completes) the element would be removed it would make easy to implement general case solution for transitions and a like stuff.
Another thing that could be needed for general case is ability to "freeze" all the internal content bindings in the right time so that in such case:
{{#error}}<div class="error">{{.}}</div>{{/error}}
internal content (which was set using {{.}}) would not change after error is not there. I don't know if it is possible.
I think some approaches could be taken from angularjs.