I chose mine because it was a good design and it was for low points. I hardly ever see anyone who has the same as mine. I think it also kind of shows I'm not some nerd who stays up on AG all night spamming game comments and rating games that I haven't played for points.
I change mine every so often, right now, in my opinion, the rabbit is the most appealing one I have. The shading is quite nice, but it's a little too common. =/
I'll probably just use this one until a seasonal armatar I like is available for use in AG3 or if they introduce custom armatars. I grown tired of changing my armatar to find one that suits me, so I'll just leave it as this gray/black shield until one of those two things happen.