Buy the other decks only if you want to play them, or are planning on keeping sealed copies for the next 5 years (or so)
Being a blue and red Commander, there are tons of ways to draw more cards and deal more damage with combos Curiosity and Niv-Mizzet, Parun, which is already a combo in itself, but now its faster
Because cumulative upkeep now uses age counters, Chisei, Heart of the Oceans can eat an age counter from a cumulative upkeep card, thus keep the cumulative upkeep cost manageable while also keeping the 4/4 flying creature alive
But while most only provide +1/+1, or in very specific cases +2/+2, Starlight Spectacular can grow much larger, and for only a little more mana
This is partly because the deck comes with the aforementioned ready-built combo involving two creatures, but this deck has a high creature count in general