Unfortunately, we often see the exact same argument used by both socialists and some of the more extreme Bernie Sanders fans today with regards to Trump.Why do you consider this unfortunate?
First of all it is not just those groups. Are you aware that the same comparison has been made by Holocaust survivors like Anne Frank's step-sister, national leaders such as the president of Mexico, and prominent conservatives like Glenn Beck? It isn't just a few socialists who are concerned.
That said, it would be unfortunate if the assertion had no basis in reality.
But does it? According to political scientists, the one trait that is most likely to predict you supporting Trump is how much you tend towards authoritarianism. This is why extremely authoritarian groups such as the KKK and neo-nazis have come out in support of Trump. Extreme authoritarianism certainly matches Hitler.
Next there is the question of truthfulness. According to Politifact, over 2/3 of Trump's public statements are lies. In fact he's more likely to issue "pants on fire" lies than to say something that is true or mostly true. (See http://www.politifact.com/personalities/donald-trump/ for a source.) Furthermore when he lies and is challenged on it, he shows no regard for the truth. Instead he repeats his lies over and over again, very forcefully. This "Big Lie" technique was a classic trademark of Hitler's.
What else is characteristic of Hitler? Extreme nationalism! Do I need to enumerate instances?
How about war mongering? Oh boy. So far Trump has promised to bomb North Korea, Syria and Isis. He has also suggested bombing Iraq and starting a trade war with Mexico to get them to pay for the wall he wants.
What about mass deportion? Hitler promised to deport the Jews, then killed them when nobody else wanted to take them. Trump promises to deport about 11 million people from the USA. How do you think that will work in practice?
You apparently think that Trump/Hitler comparisons are unfortunate. I consider it a quick way of identifying all of the characteristics that Trump has which make me afraid of the possibility of his being President.