Given the opportunity to play good, bad and grey I will try out all three paths though I usually end up with a morally grey character. In the Mass Effect series there's always been two save games I've ported, one for paragon and one for renegade. Ditto in SWToR I have light side, dark side and grey characters because the options are there and I want to see what happens.
As long as a morality system actually has consequences it can be really compelling, but that takes a lot of work and branching. Otherwise it comes off as hilarious-in-a-bad-way when you're like 'I AM THE DARKEST EVIL!' and the game goes, 'Okay! Nothing bad will happen in consequence because we really didn't expect people to pick that path!' and you're like 'Wait, what?'
SWTor minor game play spoilers
If you're talking about a military based game or a game with war it's nice to have a more shades of grey system, especially if there are opposing factions. Since in war even if the big guys have some pretty clear cut reasons for why they're fighting each other there are always little guys who are screwed.
Though there is something really compelling about writing something where you go through a side and people are like 'Hell yeah! Get those Blue guys, they're jerks who are limiting our (freedom, justice, ability to feed ourselves, ability to take their resources, culture, something else) and you're a hero!' and then you do a play through as the other guys and they're singing your praises for attacking the red guys because they're the real 'bad guys' who are (oppressing, trying to take their land, resources, freedom, justice, suppress their culture or something else).