As someone who places a high value on the integrity of the color pie, I would think this argument resonates with you @Velrun It's hard to defend a mono blue card being able to cast Cruel Ultimatum in a deck, even if you ignore the fact that you're spending 2 less mana here and you get a 3/3 creature out of the deal. The fact that it so easily breaks the color pie without any impact on your deck at all means I have to agree with sigh here, Arcane Savant is indeed a bad design.That card IS bad design.
It breaks the color pie. You can cast a WBRG spell in a mono-blue deck and benefit from it without having to deal with any color restrictions, because that spell isn't part of the deck itself. That leads into: