With all due respect, your explanation is not convincing. Telerik has recently taken the fairly consequential step of announcing the deprecation of a number of skins, some of which are certain to be fairly widely used. The stated reasons for this are numerous, but the underlying reason is that, increasingly, you are finding it difficult to support such a large number of controls in your demo code base with so many skins. This justification is perfectly valid. But here you are telling me that Telerik released an entirely new control, with all of the online demos and documentation that goes with it, just to improve the performance of a fairly narrow and limited use case of the treeview in combo combination with a large number of nodes? This hardly sounds logical.
Instead, what is logical, is that Telerik saw that many people like to use treeviews in combo boxes in a variety of cases, and Telerik decided that it would be good from a performance and ease of developer implementation standpoint to create a combined control. This makes perfect sense, and I commend Telerik for going in this direction, much like they have the new standalone AutoCompleteBox control. The problem is, in order to replace the Treeview in combo box combination with a single control and make it so developers adopt it, you need to replace the functionality of the former. Now, maybe for some obscure technical reason, it would be difficult to replace 100% of the treeview/combo functionality... but this cannot mean the exclusion of BASIC functionality like the ability to modify the treeview at runtime.
And this isn't even the only missing functionality in this control. There's an entire thread (http://www.telerik.com/community/forums/aspnet-ajax/dropdowntree/beta-control.aspx
) which deals with other missing functionalities and questions whether this was supposed to be a beta release.