Apart from moral point mentioned above I see one issue with it:

In praxis you almost never get candidate who is different enough in a good way. So you always vote lesser evil, which means you vote evil.

More over even if this candidate would be voted in he would necessarily need to do a lot of compromises (because that's the setup of the system).

So at the end you again get practically the same result, maybe with some minor tweaks or different rhetoric.

I personally would consider voting only in situations like that the candidate would be a different enough plus have real chance to change something specific which I would consider as important.

Which never happened 🤷‍♂️ where I live.

I'm not that much into US politics to evaluate if RFK would be that guy, but I doubt it.