It depends on the validate js that depend on mootool, so you need to load all the libraries to make it working.
Unfortunately your evaluation does not correspond to the truth. It depends on Bootstrap - of corse and it is clearly stated on the plugin page and documentation and uses jQuery. There is no dependencies on Mootools or validate.js. You're mixing the JProiCaptcha dependencies with your forms dependencies which are two separate things. Your forms or template might depend on those libraries, not JProiCaptcha.