In the configuration page of the toolbar control, there is an example demonstrating the usage of items.attributes.
If you run the example in the code editor (http://dojo.telerik.com/ADEDU), you will see that the "My Button" button does not render properly as a button. A look at the source shows that a few kendo classes are not added properly to the dom element, including k-button.
If you run this modified version of the example where all I changed is the version of kendo UI used, you will see that it used to work.
It appears the problem occurs whenever custom attributes are defined for items.