If you already have Photomatix Pro, you only need the Tone Mapping Plugin for Photoshop if it is important for you to be able to use the Tone Mapping methods within Photoshop CS/CC directly.
The standalone Photomatix Pro and Essentials already include Tone Mapping. Additionally, the standalone programs include Exposure Fusion, which is not available in the Tone Mapping Plugin for Photoshop.
The differences between tone mapping with Photomatix Pro versus tone mapping with the Photoshop Plugin are detailed here.