Backend validation re-checks all the form inputs to ensure they still comply with the validation rules after the form is submitted. For instance, if you have a number field that is restricted to a value from 1 to 10, we make sure on the front end that the form cannot be submitted if the value entered is 50. However, it is possible for a powerful user to change the front end code on the browser and remove this 1 to 10 restriction on the field. They could then enter any value and allow the form to submit. With backend validation, we would recheck the value against the 1 to 10 and reject the submission. Without backend validation, it is possible that a form field could be submitted that doesn't conform to the validation rules. 

If this form is an internal form, this form would be considered relatively safe as long as no internal employee turns out to be a malicious user. I would also consider the severity of what would happen IF some of the required fields were not filled out or some of the validation was not met. You could also use a gateway in the process to check that all required fields are properly filled in, and if not, send a user task back to the initiator to correct the form. If the form is public facing, I would be more cautious about turning off backend validation. 

As for the fix to this issue, we did fix a few bugs relating to what you are describing. It looks like we fixed a bug where columns in tables were not getting properly ignored when the table was hidden and fixed a bug relating to required list fields and blank values. Without knowing more details about the specific required hidden fields, I can't say for sure whether it will be fixed, but if you open a support case, we can ask support to try to reproduce the issue on the latest 10.3.1 update to verify for sure.