The reason it's an advantage for the warlock is that the warlock can take extra short rests much more easily than, say, a wizard can take extra long rests. If you can afford to rest for one hour, you can often afford to rest for two, allowing you to both recharge the ring and recover the slot. At the very least, when the party makes camp, you can take a short rest, recharge the ring, and then go to bed.
There's also the effect on the warlock's nova potential. One of the drawbacks of warlocks compared to other casters is that the warlock has little ability to "go nova" when faced with a major threat. A wizard who unexpectedly finds himself face-to-face with the BBEG can pull out all the stops and burn through spell slots like there's no tomorrow; a warlock in the same situation is stuck with the same 2-3 spell slots she always has. Turning that potential 2-3 spell slots into 3-4 is a big deal.
Good points.
I do opine that the back to back short rest scenario will be unlikely in most games most of the time (except at the end of the adventuring day which merely sets up either the night, or the next day). One just needs to read the plethora of posts here on the boards about how PCs have difficulty taking a single short rest at times let alone two back to back ones.
I totally get the extra Nova ability, but it is limited to one extra spell per double short rest making it even more difficult for the Warlock player from gaining more with the ring. How often do PCs have 2 unmolested hours to burn between encounters unless it is a single encounter day (in which case, how do the PCs know that it is a single encounter day and the Warlock should nova all of his spells)?
The versatility of the other spell casting classes is easily equivalent to that. The Cleric puts extra healing/restorative spells into the ring, so that the Cleric can focus his main spells on other things like protection or novaing. The Wizard puts extra rarely used but useful spells (like Disguise Self or Suggestion) which allows him to focus his main spells on other things like novaing.
No doubt that a Warlock can in some unique circumstances get in more than a single extra spell per day, but the other casters can mix and match the spells in the ring and save the prep of other spells. That's gotta be at least equally as useful as one or two extra Warlock spells per day.
But the problem with the Warlock nova argument is that it makes the Warlock more potent than he normally is, but that's nowhere near making a Wizard more potent than he normally is. Warlock nova-ing tends to be subpar in the first place.
A single Wall of Force in the ring by the Wizard, locking down a super tough foe while the PCs attack other foes or recover from a beating, or blocking a passageway so that 30 foes cannot reach the PCs, does so much more than a single extra single target nova (or even two in a given day) by a Warlock. Most of the non-damaging nova spells in the game get a saving throw every round, so a Warlock nova often means either just some more damage (which can be done a lot of different ways by a lot of different classes), or it means that one foe is delayed a bit. It's not really that impressive when discussing nova spells of level 5 and lower.
The real game changer spells in D&D are ones that change the entire paradigm of the encounter (Fog Cloud for attack protection, wall spells to seal off foes, mass heals to restore the party, fireball to seriously damage multiple foes, entangle/web to change action economy, etc.). Warlock (mostly) single target spells are not in that "shake up the encounter" ballpark.
Warlocks do get a few game changers (damage with the Fiend patron, Evard's with the Great Old One patron, slow once a day), but they are always bringing a few types of knives while the Wizard is bringing the swiss army knife.
From the perspective of the party, an extra spell or two a day by the Warlock is a fairly minor addition unless the adventurers are going for many days where they blow through all of their spells (in which case, they have more problems than this ring is going to help with).
From the perspective of the player, sure, this is a great item for a Warlock. I just think that due to versatility, it is an even greater item for other spell casting classes.
The Wizard with the ring can have the Cleric put a Healing Word in it so that he can wake up any unconscious PC. The Warlock can only do that at lower levels (or if he limits himself to invocations in the ring instead of spells).