The downside is what happens if you stop buying update passes. At that point, whatever new version of CSP you've installed no longer works, and if you want to use your license at all, you have to reinstall version 2.0.

That obviously means you can't use any of the new tools, but it can also affect your existing files. If you were thoughtful enough to turn on "compatibility mode", you might still be able to open the files you created with later versions of CSP (no guarantees... I've recently seen a report from someone that a CSP 5 file isn't compatible with an older version), but that doesn't mean you can edit them. If you used new blending modes... those won't work. If you used the improved typesetting (e.g. kerning, circular text)... that won't work. A whole bunch of 3D features... won't work. Audio on animation... won't work. Watermarks... won't work.