The mana costs on Beguiler have always been higher than other classes due to it's insane AoE dps capabilities, and it's ability to control an entire fight from start to finish.
The thing with Beguiler is, that if you used all of your CC abilities correctly, you will miss ZERO skillshots, while also dicking your opponent around for an extensive amount of time. The Beguiler has complete control of his opponent from start to finish.
Now, what if the Beguiler doesnt use his DPS abilities? He could cycle through each CC skill and never stop. He could hold you for 5-10 mins if we go off of just cooldowns alone.
Except!
We can make him run out of mana long before this!
With his own mana regen buff and the addition of attributes, this is actually hard to make a reality, which is why his abilities have such high costs. Now, I think the CC portion of his skills have lower costs than his damaging abilities, so it's likely that you can still permalock someone for quite some time (I haven't tested it since attributes, so I don't know if it's true anymore)
But here's the gist: Since the Beguiler has such massive control over his opponents, it was desirable to punish him for failing to land his skillshots when he was basically handed a free hit for every Piggify/MassPigify he casted. If you are wasting skillshots, it will require more to kill your opponent, if you miss too many, you run out of mana before you get to do so.
That was the idea anyways. I don't know if it's currently too taxing or not, as I haven't tested any of it in months.
