That depends. I mean, it's a bit unreasonable to assume that they shouldn't have even the slightest preference. Some, however, are even more partisan than most red or blue avatars, and then you have to wonder why they have a green avatar in the first place.
You see, there's this belief among some people that claiming to be an 'independent' makes you look smarter. Why else do you think that Jfern has a green avatar?
Some people are simply more committed to ideology than party. Others might differ from party orthodoxy on one or a handful of issues that are important enough that cause a person to feel displaced (say, abortion). And then you also get your odd-ball cases like sjkqw and Hagrid who identify with a certain label more out of feeling than ideological conviction.
I won't get much into why I switched from D to I, though I will say my decision was perhaps more slightly based on cultural factors. The state I'm actually registered in (NC) has modified open primaries, too, so I don't really have anything to lose or gain from dropping the D label.
(Plus, if I have to vote against Hillary because she chooses someone like Hickenlooper as her veep, at least I'll have some guilt off my shoulders.
)