A minivan becomes a necessity when your family hits around 5 people. You can get by with a midsize sedan but it's difficult and you're unable to offer rides to anyone. Unfortunately in the US they can't or won't sell 6+ seater variants of smaller vehicles like the Prius or Accord.

Larger vehicles like conversion vans and full size SUVs that don't seat anymore people than a minivan but get half the fuel efficiency and take up twice the space are on the other hand completely impractical.