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I'd like to add my two cents to everything said above. It's a purely theoretical point, but it's regularly confirmed in practice. 
Logistics will give you an advantage in resources, artifacts, and levels, yes. But they didn't mention that with Skilled Education, the level advantage translates into a huge advantage in stats. You can ask those who play online often – how do they feel about, say, an Argath with a Logistics specialization? The answer is clear: it's hard to outpace her, both in levels and just on the map.
Now, about having two schools of magic.
"You won't have time to cast everything in battle" – that's a silly statement. You need to combine the schools of magic so that you can cast spells.
Specifically – the Necromancer. Darkness + Summoning is an excellent choice. At the beginning of the battle, mass-cast spells. Especially – Mass Slow. And then you can summon Fire Elementals. And Phoenixes. And even put a Shield on Zombies.
For example, my Leto used to summon Phoenixes and put Vampirism on them after switching to "necromancy schools of magic." And that created an unkillable, high-initiative damage-dealing unit...
So, having two schools of magic is more of a norm than an exception for spellcasters.

Logistics will give you an advantage in resources, artifacts, and levels, yes. But they didn't mention that with Skilled Education, the level advantage translates into a huge advantage in stats. You can ask those who play online often – how do they feel about, say, an Argath with a Logistics specialization? The answer is clear: it's hard to outpace her, both in levels and just on the map.

Now, about having two schools of magic.
"You won't have time to cast everything in battle" – that's a silly statement. You need to combine the schools of magic so that you can cast spells.
Specifically – the Necromancer. Darkness + Summoning is an excellent choice. At the beginning of the battle, mass-cast spells. Especially – Mass Slow. And then you can summon Fire Elementals. And Phoenixes. And even put a Shield on Zombies.
For example, my Leto used to summon Phoenixes and put Vampirism on them after switching to "necromancy schools of magic." And that created an unkillable, high-initiative damage-dealing unit...
So, having two schools of magic is more of a norm than an exception for spellcasters.