It all comes down to options. The Force Packs will bw giving you 2 of each Objective set, so if you want to run the new Navy Vader pod, that's 20% of your deck right there. If you toss in the stuff from a single Core Set to pad it out to 10 Objectives total, you'll absolutely make a functional deck to use Navy Vader in, as an example.
But, as you may have guessed, having 2 copies of an Objective set will make your ability to draw into those money cards more consistant. Forget the mains like Vader and Luke. Think of the support cards like Rebel Assault and Force Choke. If you ran a single Core, your Rebel deck could only have 2 Rebel Assaults in it. 2 Cores makes it 4. I couldn't imagine running a Rebel Deck with less than 4 Rebel Assaults to pull your butt out of the fire at this point in the game.
Now, in the future, there could be similar Event cards that help you with clearing out Objectives, and maybe Rebel Assault becomes less useful in the long-run, but for now, if you're running Rebels and have an eye on winning a match, it's an auto-include in my opinion. 2 direct damage to an opposing Unit or Objective is just to good not to use, especially if you can drop it at an opportune moment. So yes, I would stand by taking the maximum allowable, because I'd rather have 4 of those in a 50 card deck than 2.
Of course, it's all going to come down to card randomization, and sometimes probability just doesn't go your way. I played a game this weekend with my Rebel deck where the dial was at 10, the Force was with Dark Side and I had just lost an epic Edge battle that vaporized my Home One. I had 2 Objectives in my victory pile, and Emperor's Web was on it's last hit point. I hadn't seen a Rebel Assault all game.
With about 20 cards left in my deck and an empty hand, I started my turn, knowing that it was going to be the last turn of the game, one way or another. My Draw Phase kicked in, and I knew that 20% of that remaining deck was 4 Rebel Assault cards. On the 5th card I drew, I got one. I played it on the Draw Phase action window to take out Emperor's Web and close out the game in my favour.
Now, that was a good break for me, because if that card was two more cards down in the pile, I would have lost the game. But that's the thng when you're playing any type of game with a luck-based component to it. You're always going to have really amazing blowout games where you have these amazing dream hands at the start, and some games all your Vaders and Emperors are at the bottom of your deck.
The OP mentioned observing some success with a Single Core set at league night because everyone's decks were so "meta". I would invite you to always scrutinize your wins as much as your losses. I think we can all agree that when a deck tanks, it's obvious that you need to evalutate "what went wrong" and try and troubleshoot it. Maybe you need to swap some pods out, or maybe you just made sub-optimal decisions at key points in the game. Is it your deck that needs tweaking, or is it you, the player, that just needs more practice in using the deck correctly in the game?
When you win a few matches with your deck (or list in the case of wargaming, like X-Wing), it's easy to think you've got a good grasp on the deck and the game, but make no mistake, sometimes you won not because you're so great, but because your opponent was just so bad. Whenever I play casually, and myself or my opponent mulligans a hand, we like to review that decision after the fact and discuss whether it was worth doing based on the cards. Sometimes it's obvious, like you drew into 2 Vaders, 2 Palpatines, and 2 Devastators, with no "2" Resources in your starting Objectives, but somtimes it's really tough.
Getting back to the original topic, I'm sure, over time, the second Core Set will be less "valuable" to have, as more pods come out to flesh out the card pool, but that's like saying you shouldn't bother buying the Hoth cycle because in two years there's going to be other Force Packs that have their own tech that means you don't need to rely on Speeders and Hoth effects to win the day. I got my two Core Sets for $30 bucks each, and each Force Pack will be discounted down to $11. So I look at the 2 Cores I bought ($60) as being a full playset of 2 of each Objective in the "Core Cycle". The Hoth Cycle is really "Wave 2" for me, and will actually cost about the same as the "Core Cycle" ($66).
The difference is that the Core cycle's cards are all available now, and can make a solid deck for 4 of the 6 Affiliations. The Hoth Cycle is going to take 6 months to have all the cards trickle in, so your Hoth trait decks will be less than stellar until you have all the Objectives to take advantage of the Hoth effects contained within. Ultimately, they will add more options for Navy, Jedi, Rebels, and Sith decks to work, with maybe a sprinkling of Scum and Smugglers, but for now, you'll have to wait a few months to get the full Hoth picture.
Additionally, we have the Edge of Darkness "big box" expansion coming in the summer. This will provide Scum and Smugglers the much-needed injection of cards to make those Affiliation decks finally exist. One thing to note, which I'm surprised hasn't blown up into an outcry, is that the Edge of Darkness box will only contain one of each Objective set, requiring 2 boxes to be purchased to get the doubles of everything, much like Core, although I don't think Edge will leave you with any "extra" cards you can't use.