Here's a (possibly known) trick to detect such a change using Visualforce and native Opportunity layouts. It's really about having your code "know" (and possibly execute) every time a user visits a native Salesforce layout (regardless of the object.) Hence, we could call it an "On-View" trigger.

Write a very simple Apex controller that has a constructor and one method with a future annotation. The future annotation will ensure there's no lack time when the page is loaded due to the "hidden" Apex processing you'll be doing:

public with sharing class OppHelper {
    public Opportunity opp;
    public OppHelper( ApexPages.StandardController stdController ) {
        opp = ( Opportunity )stdController.getRecord();        
    }
    public void rollupOppContacts(){
        OppHelper.rollupOppContactsFuture( opp.Id );    
    }
    @future public static void rollupOppContactsFuture( Id oppId ) {
        Contact[] contactList = [ SELECT Id, Some_field__c FROM Contact
                             WHERE Id IN ( SELECT ContactId FROM OpportunityContactRole
                                         WHERE OpportunityId = :oppId ) ];
        Opportunity opp = [ SELECT Id, Some_rollup_score__c FROM Opportunity WHERE Id = :oppId ];
        opp.Some_rollup_score__c = 0;
        
        for( Contact contact : contactList ) {
            opp.Some_rollup_score__c = contact.Some_field__c;
        }
        update opp;
    }
}

Then write a very simple Visualforce page that uses the "action" attribute in the apex:page markup:

<apex:page standardController="Opportunity" extensions="OppHelper" action="{!rollupOppContacts}" /> 

Add the Visualforce page to any/all Opportunity layouts with width and height settings set to zero.

Everytime the user navigates to an Opportunity layout the code will execute. Given that the only way a user can make a change to Opportunity Contact Roles is through some sort of "native" Opportunity layout, you're ensured that this will trigger for every such change. (The exception being if you're org already has some custom Apex/VF that allows/makes changes to Opportunity Contact Roles… in which case you could do the appropriate rollup/scoring etc.. within that.) Just to be clear, if a user makes a change to Opportunity Contact Roles, after clicking save, Salesforce brings the user back to the Opportunity layout by default,, so your code will execute again with the freshest Opportunity Contact Role data.

Lastly, it should be noted that using the future annotation means it won't be totally real time… but more so than a cron job and as mentioned earlier, the page will load without delay.

Happy clouds ~~~