While is it possible to add a many contacts, companies to a Deal and vice a versa, it is sometimes neccasary to create a field of that type that stays on the record itself. If I have a source for a deal, I need to associate that person with the deal on the left side so that I can atribute the deal to that person. If I just add them on the right as a contact there is no way to tie the reason for that person to be on the deal. For example Jon Wick from the continental reffers a client Wild Bill of Buffalo Inc. to me and I write a big deal with Buffalo Inc for 500K. I want to create a property called 'refered by' and pick Jon Wick from my contact database.
The need behind that is to be able to build a "relationship field", like in SalesForce. I think it's something important and I saw many times needs for that, which is often resolved by adding a single line of text field where you can add a URL to your contact. But it's a hack-ish, not ideal solution. relationship field (https://help.salesforce.com/apex/HTViewHelpDoc?id=overview_of_custom_object_relationships.htm&langua...) like these would be better.
