Up to you, of course.
Even though I got it free until ManiaPlanet 5 (as a Turbo owner who was in the Lagoon beta), I went ahead and bought it as well, so as to support Nadeo's efforts to continue working on ManiaPlanet.
I also really love having Lagoon as part of ManiaPlanet, because of the added blocks there, and the ability to use the mesh editor and embeddable objects.
Plus, having it opens up the ability to use multi-env titlepacks and make envmix tracks, which you can't do unless you own all the environments.
We all have to decide what's best to spend our money on though.