Sitefinity comes with the full suite of RadControls for ASP.NET Ajax (including RadRotator), which can be used inside Sitefinity project, so no, if you are going to develop a solution for Sitefinity (e.g. module, control, tool...) you do not need to buy RadControls.
If you are planning to use RadControls outside of Sitefinity, for some other projects, you will need to buy the controls separately.
I hope this clarifies this issue. Let us know if there is anything else we can do for you.
the Telerik team