I never found loading another RW or ILS into the secondary that useful. It is just as easy to select another runway or ILS. Airbus procedures published by airbus have you build the route to the alternate in the secondary on the initial setup. In the US I don't think I ever used my planned alternate. Once again, a waste of time.

It probably has its uses. The only one I found was keeping a copy the original flight plan in case you make changes.