As with Bernie-or-bust types, I have little to no respect for people like this. At the very least, on the most basic level, a president appoints the Attorney General, who heads the Department of Justice. If your focus is on criminal justice or voting rights or civil rights it strikes me as very important to have an Attorney General in place who is not antagonistic to your causes. How would a President Mitt Romney and his appointed AG handle something like Ferguson? Eric Holder deserves a lot of credit for taking his job very seriously and doing everything in (and perhaps outside) his power to fight for criminal justice reforms, voting rights, (some, not all) civil rights, etc. How can someone say the president doesn't matter give the stakes of issues like these?
But as with Bernie supporters, there are a lot of activists of all stripes who believe presidents can wave a magic want and fix the country's ills, and thus the fact that ills still exists must prove the president is not an ally. The only thing presidents wave magic wands to accomplish with absolute power happen thousands of miles away from the United States, and often end in someone being blown up.
I wish all this activism would result in state prosecutors, judges, state legislators, etc being targeted for change but that remains to be seen. And just as Bernie's supporters won't spark some monumental grass roots change in local politics, I don't expect most activists to either. In my experience most people who complain the most about "the system" are not interested in changing it, making it very hard for genuine activists to organize people.