Sorry for the messy translation lol :D
 I'm adding the link to the petition Zapdos_Smithh started to get this change implemented in the future:
In SCBW and W3, units move while avoiding friendly units,
but in SC2, units move while pushing their own friendly units. (when on hold position, they avoid each other)
This is where units tend to clump up, and this happens even when you move them just a couple of times.
Units also clump up in small numbers as well.
If you watch SCBW games, units don't push each other around, so the space around each unit is always changning, and this results in a dynamic movement.
If you only move with one control group, units show the same dynamic movement.
I think this is very important for e-sports.
If I had to list the advantages of removing the clumping of units:
1. (the most important) Unit movements become more dynamic.
Even if you hotkey them separately, those small groups will move like a clump of jelly anyways. It doesn't look natural, and your army looks smaller than it actually is.
2. It's easier to tell the difference between units.
SCBW is in 2-d and it uses less colours, while SC2 is 3-d and uses a variety of colours. So it's easier to differentiate units in SCBW. You can tell this by playing SC2 in the lowest graphic settings.
The lowest graphic setting allows you to tell the difference between units and the difference between units and map tiles much more effectively.
3. You can weaken the power of splash units.
If you look at the units that do splash damage,
siege mode tanks, thor's air attack, ghost's EMP and nuclear missile, raven's HSM, colossus, high templar's storm, infestor's fungal growth,
most of them have been nerfed since beta.
One reason why they are so strong is because units always move together in clumps.
Some people will tell you to separate your units beforehand, but they clump up again when you move them around to attack.
4. The firepower decreases and the duration of battles increases.
One of the main issues with SC2 since the release was that the battles didn't last very long.
Along with complaints such as 'the maps are too narrow', 'the game time is too fast', I think this is a legit explanation as well. This is why we should change the tendency for units to clump together.
If we end up getting rid of this issue, or units move more like in SCBW and W3, we probably need to tweak the balance again.
However, if balance is something that needs to be gradually fixed, this issue should be something we need to change right now.
As a SCBW and SC2 player, and an e-sports viewer, I write this to plead my case.