Maybe from the american point of view.
From the German point of view i don't see why we would need Nato if our military is around the same strength as Russia.
First of all, spending the same amount as Russia isn't the same as being equal to Russia in military power.
Secondly, you would need NATO because it promotes stability, peace, and the fact that if a war between Germany and Russia did erupt, less Germans would die as they would have a coalition backing them up.
Thirdly, if America cut military spending or pulled back on commitments to German defense, Germany would have to increase its spending anyway. You are saying why do we need to spend money on ourselves if the Americans will just do it for us, which isn't really fair.
Lastly, I'm not saying the 2% target is actually the most reasonable point to be at. But according to this article even Merkel herself said Germany should and will increase defense spending.