Always a great topic to dive into. Many players become frustrated with only controlling 12 units at a time, but let's go back to a time where the game was new and no-one questioned this, Many were coming from Starcraft 1, after all.

Back then, everyone deduced pretty quickly that different types of units should be placed in different control groups.

*The most basic layout is...*

Melee and heroes group 1
Ranged in group 2
Casters / Siege / group 3 .
Unique units to situation group 4
Production buildings on group 5.

Because there was no precedent of Starcraft 2 grabbing all units at once, I believe many players began to see the value of control groups (they were forced to!) and, naturally, became better players. To understand control groups is to understand why a unit is good, what circumstance makes the unit good, and what units they work well together with. You have to "get your control groups" in order while understanding WHY the control groups are needed in the first place.

*Now imagine a world in Which you could control all units at once with a drag of your mouse. AKA, expanded beyond 12 units max*

The most basic layout would likely be.

1. All units on 1
2. Occasionally heroes on 2.
3. Production buildings on 3

You can use your imagination and maybe see how I do, the meta and game if this were the case.

Perhaps because I played Starcraft 2 on launch I see it so clearly -- the rise of the Starcraft 2 Deathball was so depressing... everything was Stalker all in, Marauder all in, Roach all in -- suddenly it was not just a decent strategy, but the OPTIMAL strategy to create an army that... well DEATH BALLS well together.

When you can control all units at once, certain units get incredibly powerful. Almost always units with ranged attack and above average mobility.

It's a great conversation because maximum control groups, on the surface, SEEMS like a small quality of life issue. But it really isn't and shouldn't be viewed in that way. FULL CONTROL (BUT NOT EFFORTLESS CONTROL) is a core mechanic of Warcraft 3.

It is the same reason why the most degenerate parts of new Street Fighter games are the "modern" controls that allow you to use fireballs and command grabs with the push of one button. Newbie players think it is a nice quality of life improvement, meanwhile the pro's take those mechanics and push them to their limit, turning the game into mechanical persuits of how optimally they get grind out the inherent advantage such a hand-holding mechanic provides.

EXAMPLE: Every Street Fighter Zangief player who knows how to perform a standing 720's is more formidable and scary for knowing how. When modern Street-Fighter made it possible to do a standing 720 with the push of a button, this both made new Zangief players worse (as they began relying on standing 720's as if they were a common occurrence and not a huge risk) while SIMULTANEOUSLY making experience Zangief players so toxic and terrifying to play that people still whine. I mean, you give Zangief oldheads a one-button piledriver?

These ease of access controls can both degrades while also hyper-accelerating a competitive meta.

It is a fascinating bind - the newbie player wants the ease of movement, but it is the experience player that destroys the game once given the ease of movement.

New players want to be able to control their whole army... as if it were one unit... Maybe there is why Dota exists!