“Death SELLS!” 

Yes. Though I wonder if the proliferation of character deaths has blunted this somewhat. Marvel recently had a story where Ms. Marvel died and no one thought it was permanent.

They turned Hal Jordan evil, killed him off for what seemed like a blink of an eye, turned him into the Spectre, then made a retcon/excuse that let him off the hook.

One of two is one of my absolute favorite heroes ever, and one is a hero whose book I read mainly when he shared it with Green Arrow. I've always found Hal Jordan boring. 

I started reading SPIDER-MAN in the mid-70's, at least a decade after both Betty Brant and Liz Allen made their last appearances.  (There were also plenty of Stan Lee/Ditko & Romita to read, so I knew who everyone was.)  I was surprised when they both returned!  The Liz Allen/Harry Osborn and the Betty Brant/Ned Leeds romances all made sense and were fairly important in the whole scheme of things.  And neither would have been possible if they were killed off.

Steve Gerber was a master at it.  For a while there, Jack Norris was the heart of DEFENDERS.  Same for Richard Rory and MAN-THING.  I really cared when Trish Starr lost her arm.

Other writers also get it.  It was a pleasant surprise when Dave Cox showed up again in CAPTAIN AMERICA, decades after he was last seen.  Moira MacTaggart always seemed an essential part of X-MEN, and of course Alicia for FANTASTIC FOUR.  Is Alicia doomed to one day be killed off or be given super-powers?

Dear lazy writers: If you don't know what to do with a regular person supporting character, just let them wander off.  A better writer will one day show up and make something good come of it.