Flexibility, X spells, mana reliability, blah blah blah
Most combination decks had at least one card they couldn't win without, many had multiples and even the cards they could win without could slow things down quite a bit
Its survivability means itll stick around until you can drop a Sporesower Thallid or Sporoloth Ancient to kick it into high gear
This includes treasures like Artifacts and Equipments but Creatures too
Etali is a brand new card, and if it doesnt get a reprint for a while it could go up in value
As a 4/4 flyer with infect, it can stack up poison on a single opponent fast, especially combined with his cheap haste-granting ability, particularly useful when Skithiryx is rising from the command zone multiple times a game