ComboBox - autobind:false and MVM

HTML

<link rel="stylesheet" href="http://cdn.kendostatic.com/2012.3.1114/styles/kendo.common.min.css">
<link rel="stylesheet" href="http://cdn.kendostatic.com/2012.3.1114/styles/kendo.default.min.css">
<script src="http://cdn.kendostatic.com/2012.3.1114/js/kendo.all.min.js"></script>
<div id="storeInfo">
    <input id="gradeCombo" data-role="combobox" data-bind="source: source.list, comboboxValue: source.gradeValue, comboboxText: source.gradeText" />
   
    <div id="tgrid" data-role="grid" data-bind="source: source.list, selectable: source.gradeText"
         data-columns='["label", "id"]' ></div>



</div>

JavaScript

$(function(){
    var kendoViewModel = kendo.observable({
        source: {
            gradeValue: "A",

            gradeText: "Single, Cell",
            list: [
                { "label": "Single, Cell", "id": "A" },
                { "label": "Single, Row", "id": "B1" },
                { "label": "Multiple, Row", "id": "B2" }
            ]
        }
    });

    $('#gradeCombo').kendoComboBox({
        dataTextField: 'label',
        dataValueField: 'id',
        autoBind: false
    });

    //custom binding for value
    kendo.data.binders.widget.comboboxValue = kendo.data.Binder.extend({
        init: function(widget, bindings, options) {
            //call the base constructor
            kendo.data.Binder.fn.init.call(this, widget.element[0], bindings, options);

            this.widget = widget;
            this._change = $.proxy(this.change, this);
            this.widget.bind("change", this._change);
        },
        change: function() {
            this.bindings.comboboxValue.set(this.widget.value());
        },
        refresh: function() {
            var widget = this.widget,
                value = this.bindings.comboboxValue.get();

            if (widget.dataSource.view()[0]) {
                widget.value(value);
            } else {
                widget.element.val(value);
            }
        },
        destroy: function() {
            this.widget.unbind("change", this._change);
        }
    });

kendo.data.binders.widget.selectable = kendo.data.Binder.extend({
    init: function (widget, bindings, options) {
        //call the base constructor
        kendo.data.Binder.fn.init.call(this, widget.element[0], bindings, options);

        this.widget = widget;
        this._change = $.proxy(this.change, this);
        this.widget.bind("change", this._change);
    },
    change: function () {
        this.bindings.selectable.set(this.widget.selectable);
    },
    refresh: function () {
        $(this.element).kendoGrid({ selectable:...