That is an interesting question.
So far, there will be no difference nor any advantages. However, based on useragent string, developers can toggle the simple rendering mode on or off.
In addition, this the first of many steps that Telerik is taking to make the controls much more mobile friendly. Now I can't discuss particular features, as those are still yet to be determined, but you can see KendoUI as an example what can be achieved with mobile friendly approach.
One last thing -- DropDownList, being a part of .NET, is updated with the framework, which doesn't happen that often.
RadComboBox, on the other hand is potentially updated every a couple of months and new features are available some what faster.
To sum up -- so far there isn't any distinctive advantage of choosing RadComboBox simple rendering instead of DropDownList (apart from switching modes dynamically). However, that's going to change.
I hope that answers your question.
the Telerik team
Consider using RadControls for ASP.NET AJAX (built on top of the ASP.NET AJAX framework) as a replacement for the Telerik ASP.NET Classic controls, See the product support lifecycle here.