Level 5 is important because it allows them to test:
1. extra attack for weapon using classes
2. level 3 spells for magic type classes
It is by far the largest spike in power that you will obtain in your characters career.
In most cases you are nerfing your character if you multiclass before you reach level 5. You will *eventually* hit that power threshold, but you delay it by however many levels your multiclass dip lasts. This is often a bad idea.
From Larian's perspective, if they actually want to test the balance of their encounters then they NEED to allow us to reach class level 5 (not character level 5). Anything else is just more faffing about.
