I'm not going to look up exactly what Mewtwo said to Pikachu after the car crash, but he says something on the lines of "if you let me do this it will save him". I would like to think, just like a pokemon has the will to break free from a capture, (like in the beginning of the movie) it would have the will to break the boding of another human. The reason they needed to us the R was to make the pokemon more susceptible to Mewtwo's bonding. Also He had to do it on a large scale which would take a lot of effort, the pokemon had to be in a more susceptible state. We also know that R lowers a pokemon's ability to make good decisions and be more reckless.
Pikachu would obviously risk himself to save his partner because they're a team. Mewtwo doesn't need the R to join 1 pokemon and person, but we can all see why it would make it easier.