How important they are varies from case to case, of course. Lind cites Lee Harvey Oswald's apparent attachment to Marxism, and it's a valid point. In Oswald's case, the actual evidence on his particular mix of motives is vague, even apart from the fog of conspiracy theories that bloomed around the John Kennedy assassination. (And, yes, I'm one of the 10 or so people in the US who believe that Oswald acted as a "lone gunman.")
Lind writes of another famous Presidential assassination:
The most famous presidential assassin of all is John Wilkes Booth, and there has never been any doubt as to why he killed Abraham Lincoln. Booth was a racist, pro-slavery supporter of the Confederate States of America. Like Oswald, Sirhan and Czolgosz, he saw himself as a revolutionary in a larger political cause: white supremacy, which was threatened by the triumph of Lincoln's "Black Republicanism" over the Southern secessionists. Unlike the other assassins [he previously discusses], Wilkes was part of a larger conspiracy, which included an attack that injured Secretary of State William Seward around the time that Lincoln was murdered.In the case of Jared Lee Loughner, who attempted to assassinate Gaby Giffords, the political motive might have been small, but is still important to understand. As Dave Neiwert observes in Right-Wingers Use Boston Bombing to Paper Over Their Own Extremist Terror Orcinus 04/23/2013, "Loughner was carrying out what he saw as a mission on behalf of his now-adopted right-wing beliefs involving a global monetary conspiracy. He was indeed a right-wing extremist, and other experts on the subject who have examined the record have reached the same conclusion."
The accused Aurora mass killer James Eagan Holmes had no political motive so far as I know.
People who don't want to promote anti-Muslim bigotry are justifiably cautious about seeing pundits and propagandists hyperventilating over the Tsarnaev brothers' apparent adherence to radical Islam. And conservatives are understandably leery of having violent rightwing killers associated with conservatism.
But the political motivations of actual and would-be terrorists and mass killers is not something that should be airbrushed out of the picture if it's really there.