Rescue ranger is there to help improve moving up, as a whole, as well as being able to heal with no metal. Its not there to be super aggressive. Especially not in the competitive world, which is what I see and play. In the competitive world running a wrangler/RR means your sentry gun can waste an entire uber, and have a chance of the sentry not actually dying. It means you can hold out forever. Having a pyro play WITH you, means if he's running the homewrecker he's going to not only remove the spy from play, but he's going to save your sentry and prevent the enemy team from pushing without uber or without a sneaky play from demo/soldier. The RR is perfectly balanced at this point, and adding an extra bonus to it would either force valve to add an extra negative effect to it, or force UGC to ban it, and change the entire metagame of pushing times and sentry placements.