If you skip the messages parameter, the plugin will display default, auto-generated messages.
However, You should always customize the validation error messages.
Here you can see screen captures from Firefox and Chrome: Text alert messages are generated entirely by the browser and will even translate automatically into different languages - something that would be almost impossible using just Java Script.
The advantage for the user is that it's obvious whick element is causing the problem and there's no alert window that needs to be clicked away.
All of this was from the Google API notes : Google Recaptcha v2 API Notes I'm a bit unsure why you would want to do this.
Using a library to do form validation can save lots of your development time. j Query Form validation library is the most popular validation library.
This post collects all my notes and references on j Query Form validation library.