I have to respectfully disagree with you on this one.  In a perfect world yes
there would be no need of sealer when putting in o-rings.  That is thier job
to act as a seal.  However when installing o-rings on a older vehicle there
could irregularities on the surfaces do to corrosion or even scratches that
will not allow the o-ring to seal properly.  Instead of putting the whole
waterpump back on installing the waterlines and then filling the system only
to have it leak because of some pitting in the block or some other cause, I
recommend putting a thin film of the blue RTV on the o-ring.  This acts as
cement to hold the o-ring in place while you are installing the pump (they
fall out of that groove pretty easy otherwise) and also seals any
irregularities that might be in the sealing surfaces.  I call this cheap
insurance.
Please no flames.  My asbestos suit is at the cleaners :)
Ken Wilford
Van-Again
John 3:16