Resting is one of those mechanics that, on its own, doesn't offer all that much to a game.
HOWEVER... once you add that level of realism, it only makes sense to keep adding layers to it.
For instance, if you can just sleep after every fight with only a marginal chance of getting attacked, why not do it all of the time? Okay, so that means you should add a food/hunger mechanic, where if the party doesn't bring a metric ton of foodstuffs with them, they can't afford to rest all of the time or they will be hit by starvation penalties. But if you can just buy a ton of food all the time, that just seems like a easy work around, so you'd need to have a weight system, since there should be a way to prevent the party from having all the food they could ever need for the entire game (and then some) carried around at all times. But if you build in a weight mechanism, you'll have to balance game economy to this, since now you can't pick up every piece of waepon or armor you come across to sell, which means there will be less money, or you will spend time going back and forth between a dungeon, taking pieces at a time. Which means you will have to address disappearing corpses/loot and respawning enemies...
...the further down the rabbit hole you go, the more detailed it can become. Which is great - but only if the mechanics are done well AND well explained. If you try and plan ahead by buying a lot of food right up front, but find that you don't have enough gold to buy equipment that you thought you could live without for a while, or you can no longer bribe the orc at the first bridge outside of town and have to fight him (and get your butt handed to you), then that's a bad experience. Good mechanic, don't get me wrong... but a bad experience.
And walking the tightrope of explaining all of your complex mechanics within the logic of the game could be viewed as hand-holding to many (which they don't like) but if you don't explain anything at all and have the players learn by failing, then that can result in rage-quit and people giving up before they ever really had a chance to start.
To get a little more on topic, if PE is going to use a cooldown system, I'd prefer it be tied to a particular skill branch or magic school. So if you cast a spell from a particular school/branch, it would have a cooldown to use that particular skill/spell again, but it would also put a 20% cooldown time and mana cost on any other skill/spells of that same school during the encounter. After the encounter, these cooldowns reset (but mana does not auto-regenerate).
Example: Mage McWizardpants has four spells from three schools: School of Destruction: Fireball (10 second cooldown, 5 mana) and Ice Storm (20 second cooldown, 15 mana), School of Creation: Grasping Vines (10 second cooldown, 5 mana) and School of Spirit: Curse of Pantsfalling (5 second cooldown, 3 mana). McWizardpants has a total of 100 mana entering a fight.
He starts off a fight with a fireball, which costs him 5 mana (going down to 95 mana left) and has 10 seconds before he can cast another fireball. He then casts Grasping Vines for 5 mana (down to 90 mana). He wants to use another attack spell, but his cooldown of Fireball isn't over yet, so he uses Ice Storm. Now, since he has already cast Fireball, its going to increase the cooldown and the cost by 20%. The base of Ice Storm is 20 seconds and 15 mana, so that will now become 24 seconds and 18 mana (which takes us down to 72 mana). He casts Curse of Pantsfalling, which only costs him 3 mana (down to 69). His fireball has now had a cooldown completed, so he casts it. However, since he has already cast two spells from the School of Destruction, it will have a 40% larger cost and cooldown. The base is 10 seconds and 5 mana, so this would mean the cooldown is 14 seconds and the cost is 7 mana (down to 62). His enemy has escaped the vines he cast before, so he casts Grasping Vines again, but at a 20% higher cost and cooldown, making it 12 seconds and 6 mana (taking him down to 56).
While I know "cooldown" is considered a vulgarity to many people, if PE is going to be using it, I could see this working. It would encourage wizards to have (and USE) a wider variety of spells from different schools, rather than just spam the attack spells constantly. It would really showcase the support role of the mage, and encourage him to keep his attack and personal defense spells on the back burner in case an enemy breaks through the ranks and puts him in direct harm.
The drawback to this system is that, at low levels, this really penalizes the player to being able to cast spells very few times, or requires chugging potions (since cooldown and cost for magic missile would be through the roof by the end of a fight if that's the only spell you have). Also, potions would need to be expensive and/or rare in a case like this, otherwise the cost is negated (although the increased cooldown each time a school is used would prevent outright spamming).
This system could add to the character/level building mechanic, by having the ability for a wizard make a school his "specialty", which could do something like reduce the cooldown/cost increase to just 10%, instead of 20% (for that school only). It would be an interesting way to offer more conscientious spell casting and abilities, instead of ignoring the vast majority of spells and going for the highest DPS build.