Some third-party host applications still do not support VST3 - either at all, or in still useful older versions. Other third-party hosts that support VST3 are more robust with VST2 plugins.
Steinberg hosts work better with VST3 versions of Steinberg plugins, as you would expect from the creator of the VST3 standard. There is little advantage in not installing a VST2 version as the disk space saving is tiny - most of the disk space used is the content, which is shared between all installed versions of the plugin.