I was wondering if it was necessary to wield spells in my offhand and daggers in my main hand to be an effective nightblade. I'm currently using spells with my right hand and daggers with my left hand. Problem is, the left hand attack animation is slow, and swings in one direction.
Your main hand swings a weapon about 15% faster than your off hand. There's no difference for spells, so on that basis alone it's better to keep a weapon in your main hand.
Another advantage of keeping magic in your off hand is that you can use spells (like Flames or wards, for example) or keep them charged while non-power attacking.
For spellswords I'd really advise putting magic in your off hand. You can be effective however you'd like because the game isn't that hard, but there are really only downsides to moving your weapon to the off hand.