I really believe you are missing out HUGE if you haven't yet subscribed to the monthly subscription.
I honestly believe the $10 monthly subscription to have the Summoner Supreme status is one of the best things that happened to the game.
5* shards you save
Just from the amount of 5* shards I've saved since I first signed up when it was first offered about 3 months ago is already well worth it. I typically get around an average of 12K 5* shards every two weeks from all sources so since last July I've purchased the 5* featured from the Black ISO store for only 11K shards (if you are Cavalier status). So without the monthly subscription, it would not be possible for me to open 5* featured twice a month. I am saving 6K 5* shards per month and am able to open 5* featured instead of the basic 5* crystal. I believe most Kabam 5* offers that exceed 5K shards costs more than the 6K 5* shards you save from buying in the Black ISO store.
Energy requirements
Before the monthly subscription, I normally have to spend 300-600 units for the full energy refill to explore the Heroic, Master and Uncollected monthly events (100%). Now, I am able to explore all three difficulty levels and still have a week left without using any units to purchase energy refills. Assuming you spend even 300 units for energy, that's already the cost of the monthly subscription.
Increased Inventory Capacity
This is such a bonus as well because you can stock more of the stuff that usually ends up in your temporary stash that expires after a certain period. Definitely very helpful because I no longer have to use or sell resources for the sake of not letting them expire.
Extra Gold
The weekly 100+K Gold is superb because it helps in ranking those expensive 5 rank ups. And the Gold Deadpool isn't hard to beat at all.
Test of the Cavalier bonus rewards
This adds 700 5* shards and 75 units every 7 days (= 2800 5* shards and 300 units per month) by just doing the usual stuff you do (leveling up, questing, doing arena, etc.) I believe the stuff you get from this bonus rewards is already worth the subscription price.
Whatever else the future holds
From the survey sent to those who have subscribed, it is not difficult to imagine that some other benefits or perks will be added to the Summoner Supreme status. Should some other well thought of benefit be added then all the more should you consider signing up for this monthly subscription.