You're correct that the elites from the Core and Hoth boxes are generally worse than the regulars for their cost if you are actually paying threat to deploy them. They exist mainly for skirmish and as options for initial and reserved groups that you don't have to pay threat for. The only figures from those boxes that I ever consider the elite version is the Probe Droid, as the damage output increases significantly with the re-roll and better surge abilities, and the +2 health is quite useful on a figure that can heal itself and the E-Web, as the +1 block and increased damage and healing is a big upgrade to the very squishy regular E-Web.
Another reason I can think of wanting to spend threat on elite figures instead of extra figures is if you are using attachments and want to pile a bunch of attachments onto a group and make it harder to kill that group.
As others have said though, there are elite figures in some of the expansions that are worth paying the extra threat for, although generally I find it's more useful to just flood the board with as many figures as possible unless the Rebels have Fenn or Drokotta. Off the top of my head, elite Jet Troopers, Gamorreans, ISB Infiltrators and Sentry Droids offer significant buffs over their regular versions to make them worth taking.