[SalesForce] ActionSupport with lookup field: is ActionRegion required

I have a VF page based on the Case object. I have the standard contact lookup field on this page. When the user enters a contact, I would like to retrieve immediately the contact mailing address fields and do some background calculation with the data before the form is submitted.

I know this works using a combination of ActionSupport and ActionRegion. See code below.

My question: is ActionRegion is required for this functionality? I only need the related contact data for background calculation, not for display. I want to avoid using ActionRegion because it forces the user to enter data in required fields in other parts of the page form.

However, when I remove ActionRegion, it looks like ActionSupport does not execute when I submit the VF page form. In the submit method, the related contact data is always null.

Is there a way to do this without using ActionRegion?

Here is the code pattern that I am using. I found this on Bob Buzzard's blog. http://bobbuzzard.blogspot.com/2011/11/retrieve-related-object-fields.html

    <apex:page standardcontroller="Contact" extensions="RelatedController">
 <apex:form >
   <apex:pageMessages id="msgs"/>
   <apex:pageBlock title="Contact Create/Edit">
      <apex:pageBlockSection title="Contact Information">
       <apex:inputField value="{!contact.FirstName}"/>
       <apex:inputField value="{!contact.LastName}"/>
      </apex:pageBlockSection>
       <apex:actionRegion >
          <apex:pageBlockSection id="accinfo" title="Account Information">
         <apex:inputField value="{!contact.AccountId}">
            <apex:actionSupport event="onchange" action="{!AccountPopulated}" rerender="accinfo, msgs"/> 
         </apex:inputField>
         <apex:outputField value="{!contact.Account.AccountNumber}"/>
         <apex:outputField value="{!contact.Account.Site}"/>
        </apex:pageBlockSection>
      </apex:actionRegion>
      <apex:pageBlockButtons >
        <apex:commandButton value="Cancel" action="{!cancel}"/>
        <apex:commandButton value="Save" action="{!save}"/>
      </apex:pageBlockButtons>
   </apex:pageBlock>
  </apex:form>
</apex:page>

public with sharing class RelatedController 
{
    private ApexPages.StandardController stdCtrl;

    public RelatedController(ApexPages.StandardController std)
    {
        stdCtrl=std;
    }

    public void AccountPopulated()
    {
        Contact cont=(Contact) stdCtrl.getRecord();
        cont.Account=[select AccountNumber, Site from Account where id=:cont.AccountId];
    }
}