I think, you all confused what purpose [Heroic] missions really serve.
- It is not part of planet story
- It is not part of class story
- It can be skipped without decrementing to your experience
= You aren't required to do them in any way or form
+ It giving fun alternative to players completing content in group
+ It provide one of a kind orange gear as reward or planet commendations in ample quantities, so you can feast yourself at commendation vendor
+ It sometimes giving you unique loot from inside heroic NPCs, like orange bracers/belts (I look at you, Taris)
= Right alternative to solo questing or just bonus activity with right rewards for right kind of people.
At level 50 you also have few daily heroics, and all of them giving you just that - bonus. Be it purple mod or armoring, or one of a kind orange gear for your companions, or commendations in ample quantities, so you can feast yourself at commendation vendor... see the trend here now? Guess no one have problem finding group for level 50 dailies, right?
TLDR: It is bonus - you can do it and get above average rewards, or you can skip it easily.
P.S. I've completed almost all [Heroic] missions on every planet, and never I met problem finding group for them. Even while at average planet population is 15-20 on my server.
In fact I have had bigger problem finding group for Ancient Probe droid on Hoth, then for any of Hoth bonus series heroics.
15-20 people on a planet is a lot I hardly ever see more then 3-4 people on a planet.