To me, adventure games are all about the satisfaction of solving puzzles and slowly progressing through the story as you do so. Watching a playthrough of an adventure game spoils everything that makes them worth playing in my opinion.
While I wouldn't start by watching a playthrough, I disagree about the rest. I've completed several adventure games time and time again even though by that time I knew the solutions to all the puzzles. Good adventure games are not just about puzzles, but about the story. If the story is good, and especially if the humor is too, it's worth playing through more than once. I don't remember how many times I've played these games, but it's a lot: Loom, Monkey Island 1+2, Day of the Tentacle, Beneath a Steel Sky, Maniac Mansion, Zak McKracken, and probably several others that I don't recall at the moment.
I never actually played Grim Fandango. I tried it briefly once on a friend's computer, but I must admit that the shift in controls from the previous LucasArts adventure games threw me off.
And especially not Grim Fandango! It really doesn't need a sequel.
Why? Telltale is the only AAA studio that knows how to make good adventure games. Walking Dead, their last game, won the most GOTY awards last year.
That is irrelevant. Not every game or movie needs a sequel, regardless of who were to make it.