The activate is the "action key" of Blockland. You could probably make a quick keybind to holster your gun if you have one out, click, then equip your gun again. It would be nice to have more support for keybinds based on the needs of add-ons, though.
Since activate was introduced in v9, it wasn't implemented for vehicle entering or item collecting, unfortunately. We have add-ons for those, though.
If jet wasn't established as the primary use for right click, it's unfortunate we can't use that for a "secondary fire" trigger. Considering the number of weapons that contain extra features, a secondary fire button would still be nice to have.