They could have expended more resources to annihilate the goblins and maybe gain a few more cp. But annihilation of every foe is not the goal- winning (by methods including avoidance) is enough.
But if you use all your resources on one fight and the rest of the goblins take off while you rest and you still accomplish your mission, you've succeeded. You completed your goal and you "avoided" the rest of the encounters by making them run away.
If they take the plot item with you, and you need to track them down, it just means you get to fight them all at once the next day when you catch up with them, this time with full spells(and them all close together for AOE spells). It's likely you'll even have the resources to "waste" spells scrying on them and teleporting right beside them to catch them by surprise while they sleep.
That makes the 15MAD a good tactic.
The problem is, whether those extra encounters materialize really IS the point. If the PCs never see a second encounter(either because they've run away after one encounter to rest, or because the rest of the encounters leave while they are resting, or just because the DM doesn't feel like running any extra encounters that day) then it's tactically wise to use all your resources in the first fight.
The only time it becomes a bad idea is if you know you are going to fight at least 4 encounters a day. Most combats are balanced around the PCs using about 1/5th of their resources. Which means, even if you use "all" of your resources in the first combat, you'll survive another encounter with not much problem. Most of the time, it's impossible to use ALL of your resources in one combat. Instead, you just use your highest level spells(since you are still limited to one(or two) spells a round, you can't possibly use ALL your spells). This leaves your medium level spells for another encounter, and your low level spells(plus reliance on the fighting classes in your group) for the 3rd encounter. After that, you'll have to rely entirely on your fighting classes, often causing you to lose.
This means that even if you "go nova" during an encounter, the DM needs to throw at least 3 random encounters at you after that before you begin to regret that decision. Plus, that number increases the more spells you have per day. I've seen level 20 groups in 3.5e be able to "go nova" with enough damage to wipe "appropriate" encounters in 1 round for 3 combats in a row before they even think about resting. Then they can survive easily 3-4 more before they regret going nova.
This got so boring to run as a DM(having a game day last 2 sessions and be nothing but combats gets kind of annoying), I just started commonly using APL+4 or +5 encounters against the PCs as average encounters. That way, even if they went nova, they still didn't defeat the encounters in one round and felt like they were being appropriately challenged. Though, if they didn't go nova before I increased the challenge, I forced them to after...since they'd lose to those encounters if they didn't go nova.
Instead, they opted to save those resources for use in possible future encounters. Whether or not those encounters actually occur is immaterial- they made a risk/reward analysis that let them preserve combat & campaign resources for when they might be crucial on a "living world" campaign.
If it were flawed, then there wouldn't be so many people who have experienced and recommended it as a tried and true remedy.
It will stop people who aren't all that dedicated to "going nova". But some players love big numbers and winning quickly more than anything. Plus, I've seen them do a "cost-benefit analysis" on the tactic.
It comes down to this: If we cast 6 of our highest level spells amongst the party of full offensive spells, and win before the enemy can even fight back then that means we:
-Don't need to use any magic items or spells to heal which are resources we have for later.
-Didn't lose any hitpoints, which are part of our resources
If the day goes long and they are forced into multiple combats, they will rely on different resources than high level spells. If it doesn't go long, then nothing is wasted.
The problem is, I've seen lots of people say that "random encounters and time limits fix this problem 100% of the time. I had a group that tried the 15MAD once, I just threw a random encounter at them and they learned their lesson and never tried it again."
When I throw a random encounter at them after they try the 15MAD, they beat the random encounter and then say "Whew, I'm glad we left after one encounter...imagine if had tried to go further into the dungeon and fought 2 or 3 more encounters and THEN had this random encounter...we would have died. Remember, we always need to pull out of the dungeon as soon as possible in the future."
The thought that they could use less spells in each combat and rely on at-will resources like the fighters weapon attacks never occurs to them at all...after all, why WOULDN'T you try to beat the enemies in the best and fastest way possible? Plus, what's the fun in playing a Wizard if you can't have the satisfaction of saying "I kills the entire encounter in ONE ROUND! If you total up the damage I did with that AOE spell, it was 800 points of damage! Bet you can't do 800 damage in a round, Fighter!"
And time limits are something I hate adding artificially. I find over 50% of adventures don't want or need a time limit. Which limits me to either ONLY running the other 50% of adventures...or finding a kind of contrived reason to add a time limit to the ones that never needed one.
I like the idea of an adventure that says "You found an ancient map to a tomb of a great wizard that's been missing for centuries. It is said to hold a fortune in gold and magic items. Only problem is, can you survive the traps and summoned/created creatures left guarding it?" To then have to add a plot element that says "Oh...after you find the map then you hear there is a magical plague that is going to kill everyone if it isn't stopped in the next 2 days. Rumor has it that the cure is inside the dungeon" just seems kind of silly to me.
I like the idea of adventurers as "making their own destiny" searching for treasure in lost places. But those adventures don't have time limits on them.