The idea that only professional politicians should be admitted to office would've incensed the left 10 years ago. These days, because Trump made it in instead of say, Jon Stewart, the left is suddenly very pious about political office.

Of course people without professional political career can be presidents, even great presidents sometimes.

However, if the main differentiating factor of your favorite candidate is that he had zero experience in politics, it's hard to take that argument seriously.

People considered Bernie Sanders an outsider, and he's been a senator since 2007. Trump isn't an "outsider": he's simply unqualified.

It depends. If you consider political experience (especially extensive political experience) to be a marker of corruption and ingrained hypocrisy, political inexperience can look good by comparison. There seem to be enough people sufficiently cynical these days to see it that way.