Why on Earth are their hands tied? I'm not suggesting they actually fight Hitler, I'm suggesting that they could fight a human, but still dangerous, bad guy who happens to be set in the past (and actually I see no reason why you can't project this into the future. I'd count a futuristic story that still didn't have "monsters" in it the same.
I'm not advocating making the whole show like this, but maybe just one episode a year, or even one episode every couple of years.
Typically, the Doctor doesn't interfere in Earth's history when there wasn't outside interference. He didn't change the Aztecs, he didn't try to stop the Reign of Terror. That's how their hands are tied.
Besides, if I wanted a pure historical, I'd probably just read/see a story set in that time. The Scarlet Pimpernel is a far better story than Reign of Terror. But, I tend to prefer SF, which is one reason why I'm interested in DW.
I wouldn't be opposed to one every couple of years. I'm just glad that they dropped the pure historicals as a common story type long ago.