RTE RSV is there to meet reg requirements. I.e. if you need 10% extra route reserve, you put it in the FMS. The FMS will then warn you if you will not meet your FINAL, ALTERNATE, and RTE RSV requirements before you depart.
However, it is interesting that RTE RSV is viewable on the Fuel Prediction page once airborne. Naturally, this number is constantly decreasing as you progress towards your destination. This insinuates that the FMS will still warn you if your fuel falls below requirement, but it will be less and less of a number to be warned of (because RTE RSV is decreasing & FINAL/ALT fuel hasn't changed). Sound right?