While quite a number of them are niche units, for every Momiji out there you have a Claudia or Solano. Okay, for every other Momiji, but you get the point.
Besides, for a free player, any event unit is pretty worth it, being free and all. Well, maybe not min-max worth it, but anything is better than nothing at all.
If all else fails, bed-warmers! XD Harem must grow!!!