Sure, but with the other factions seeming mostly to be grey, it's just kind of jarring to see what's essentially a whopping great sign saying that the Legion is evil. I mean, I can't even conceive of playing a character screwed up and twisted enough to support them.
Then you can side with the NCR against them; make them enemies (which is how I usually play it). But when you visit their camp, you see a side of them that makes them gray - they are really nice human beings just being controlled by a leader who is a bit mad with power. But they do as commanded, and always will; so if you side against them, you side against all of them.
I usually spent my visit at their camp without helping them, but did spend time getting to know them. And it bothered me to slaughter them, but I felt like it was best for the future of the Mojave; so did what I had to do (just like they would have done to me if they knew I sided against them before I popped into their tent while they slept with my anti-material rifle, lol.