Yeah, the only reason we haven't seen Rainbow and Fluttershy's parents is there really hasn't been an opportunity to really show them. They're also from Cloudsdale instead of Ponyville, so presumably their parents are still up in the clouds somewhere.
As for Scootaloo's parents, they
could have had a scene of her getting permission to go camping from her legal guardians in "Sleepless in Ponyville", but the writers opted for Sweetie Belle/Rarity shenanigans and it didn't affect the plot in any way.
Also, with the parents we
do have, we don't have a lot of information. Twilight's parents are a G1 Twilight
Expy and some blue guy. Pinkie's parents are Amish rock farmers. We don't even know if Rarity's dad is a unicorn or not and he and Rarity's mom have the most personality out of all the Mane Six parents!
Something I just realized. This season we've gotten cameos from Magnum (Rarity's dad) and
Clyde (Pinkie's dad). Think we're due for an appearance from
Nightlight (Twilight's dad)?