Not necessarily. I have two Thunder teams for example, one in slot D with Ygg and one in slot E, with Phoenix. Simply put, thanks to RNG being RNG, I don't HAVE a Light team.
Also, I've seen some people use Mii (or in one case, a level 1 N grade Orc) as a way of telling people "Look, I've something good in ANOTHER TAB, so look THERE okay?!"
So, I wouldn't judge people that harshly.
Of course, many people actually run Mii as their main Eidolon... well... umm. Well, it takes all kinds, you know?