:before_filter validate for fields not part of model

I have created a pre_validate method and put it in :before_filter in order to make sure that a custom javascript pulldown has been selected. I am using a boolean variable as a flag that turns to true when a value is selected and I pass this variable using :with and submit_to_remote:

  def pre_validate     respond_to do |format|       format.js do         render :update do |page|           if params[:time_selected] == 'false'             page.replace_html 'error', "You must select a time."           end         end       end     end   end

This way i can render the error using ajax. If there are seemingly no problems with this route, my question is this: I also have a custom validation method in my model to check if there is a time conflict, but this only gets called after I try object.save, so I was wondering if it there are any other disadvantages to putting this custom validation into a similar :pre_filter function other than only being able to render one error at once. It seems like it might be a waste to have to create an object only to have it fail. Why not check if it fails in this particular instance first?