I am a casual player and I hate singleton. There are so many cool cards and interactions in magic that can be built around. It always saddens me that if I want to join the most popular casual format, instead of getting to use my deck based around Burning Vengeance and having 4 copies of it in 60, I have to tuck it away as a single copy in an enormous pile of 99 cards.

Singleton is great at increasing variance from game to game, but greatly decreases the ability to build around a card (unless the format has means to guarantee you get to play that particular card).

If Design leans toward Limited (vs Constructed for Development), does that mean that it was the design team that came up with the Burning Vengeance and Spider Spawning draft archetypes in Innistrad? As in, you and the design deliberately crafted cards that synergized greatly with the 2 engine cards?

One of the things Design tries hard to do and then Development follows up on is making sure there are uncommon cards that are fun to draft around.