The purpose behind using spry validation is to give the user immediate feed back while filling in the form. However, Spry validation uses Javascript, so is very easy to disable by turning off javascript in the browsers settings.
Server Side validation is used to secure the forms to make sure that incorrect data is not entered.
sure, you could remove the spry validation and only use server validation, but that would take away the user friendly aspect of showing immediate errors while moving from one element to the next.