Hmmm I see. But if a solo Fighter can get one attack per round and a solo Wizard can cast 10 cantrips per round... no, that can't be right. That's way too ridiculously overpowered to be true.
AoE spells are different. A
fireball that hits a whole stand at once is different from a cantrip that can only hit one creature in the stand. And then you have to worry about spells where the AoE is too small to hit the entire stand, and how you handle that, and what happens when you use something like
magic missile. Can you kill a stand of ten orcs with a single casting of
magic missile? That's a whole lot of mileage you're getting out of one first-level slot. It's a huge can of worms, and I'm not surprised Mearls glossed over it.
Bottom line, it would make sense to be able to cast ten spells per Battlesystem-round, but somehow or other the system has to resolve them on an individual basis. Given how few spell slots you get in 5E, there's no sensible way to aggregate ten spells into one, the way you can aggregate ten attacks.
And even if cantrips were to count as attacks instead of spells for this purpose, casting 10 Fireballs or whatever per round is equally absurd. Ten gnolls can launch one arrow at the town militia in the same time it takes the Wizard to cast ten Fireballs back at the gnolls? I have no idea how they'll handle spell in the Battlesystem, but this can't be it.
Ten arrows, not one; remember it's a Battlesystem-attack, not a regular-attack.
But that's touching on the other issue here, which is that
even after you factor in the "stands versus solos" rule, stands get one-tenth as many attacks as they should. Forget spells. If I've got a fighter whose hit points and AC enable her to survive an average of 20 orc attacks, and who can reliably kill one orc per round, what happens if you pit her against 10 orcs? In regular D&D combat:
Round 1. The fighter suffers 10 orc attacks, and kills an orc.
Round 2. The fighter suffers 9 orc attacks, and kills another orc.
Round 3. The fighter suffers 1 orc attack and drops. Eight out of ten orcs are still on their feet.
In Battlesystem combat as described by Mearls:
Round 1. The fighter suffers 10 orc attacks, and kills the stand. The fighter is at half hit points and all ten orcs are dead.
If you continued out the "regular combat" example and assumed the fighter somehow survived, she'd suffer a total of 55 orc attacks* before taking down the last one! If the stand wasn't losing members, it'd be 100 attacks. As GX.Sigma points out, you should divide the fighter's hit points by 10 for Battlesystem.
But that still doesn't fix the problem, because then you have to look at what happens when a stand of orcs fights a stand of peasants. If an orc can kill a peasant in two rounds, this battle should be over in about twelve seconds--maybe eighteen if you figure the orcs need a round to get into position. In Battlesystem, though, it takes 120 seconds. That might not seem like an issue, until our orc-battling fighter embeds herself with the peasants:
Regular D&D combat:
Round 1. The orcs kill 5 peasants, and the fighter kills 1 orc.
Round 2. The orcs kill 4 peasants and wound the last, and the fighter kills 1 orc.
Round 3. The orcs finish off the last peasant, and the fighter suffers 7 orc attacks and kills 1 orc.
Round 4. The fighter suffers 6 orc attacks and kills 1 orc.
Round 5. The fighter suffers 5 orc attacks and kills 1 orc.
Round 6. The fighter suffers 2 orc attacks and drops. Four out of ten orcs are still alive, and the fighter and all peasants are dead.
Battlesystem combat:
Round 1. The orcs reduce the peasant stand to half hit points, and the fighter kills the orc stand. Five out of ten peasants are still alive, the fighter is unscratched, and the orcs are all dead.
Because the orcs are dishing out damage at a tenth their normal speed, while the fighter is blasting it out on full power, the result is still a dramatic difference from the expected outcome.
[SIZE=-2]*Or 52, if you figure only eight orcs can crowd in to attack the fighter at once.[/SIZE]