I greatly enjoy the FATE system, and it's perhaps my favorite system that I've ever come across for a couple reasons. (This is referencing the FATE core system in general as well as the DFRPG.)

1. The FATE system is rules-light, but offers a very large room for modular improvements. For people who prefer more "crunchy" systems, you can basically just insert a custom system into it with little to no fuss.

2. It allows players to come up with extremely narratively different ideas in conflicts, that I as a GM am more able to facilitate. For example, I had a group going through one of the pre-gen cases and he was a minor talent, but had little to no combat ability. They got into a physical conflict so he was unable to do much on his own, until he had the idea to use the car as a weapon. In more numbers heavy systems, I would've had to do massive amounts of mental calculations to figure out how much damage it could do to feel impactful but not be broken. In FATE I just say all right roll your Drive and it's a Weapon:4.

3. It allows for character archetypes and ideas that aren't strictly the most powerful in physical conflict. Pathfinder and D&D often have this problem where if people don't build powerful combat builds they won't be useful in the encounters. Now there can be instances where a GM can facilitate a game without this problem by having it be intrigue, but I personally don't have the skill nor the players to be inclined to do it. In FATE being a know-it-all about random topics can be used mechanically in combat to help your allies. This gives people who are not necessarily all about combat the ability to still function in physical conflict. In FATE it would be entirely possible to play a complete pacifist and still contribute in fights.

If you read through a lot of the books, circles that are being used just for basic circle properties (such as blocking out things made of ectoplasm, separating the magic of the area inside from the magic of the outside for Thaumaturgical working, and acting as a cage for supernatural beings from summoning) can be used no matter what the circle is made of.

The issue starts cropping up because of three things. One is practicality. Circles that are made by nothing but some scratches in the ground is really easy to disrupt by just environmental problems. Harry mentions in one of the books that it would be incredibly dangerous if debris came by and brushed away the circle in the middle of a working. The second has to do with making circles more powerful on their own. When circles are made from certain materials they function better in certain circumstances. Certain objects and materials in the Dresdenverse are shown to have power independent of belief (Iron hurts the Fae with or without belief, White Vampires can't deal with objects touched by True Emotion etc). When these objects are worked into circles the circles function better against those things either protecting what's inside or trapping the beings inside.
The third is something that Harry only hints at a couple times, but it has to do with the way that the circles seem to work. (This is almost complete speculation but considering the way that the magic of the Dresdenverse seems to work more as a fundamental force like gravity and light some of this might make sense.) They function by taking the initial investment of energy and constantly running that energy on the ring of the circle. To overcome the circle, a being must be able to overcome that energy and destroy the "wiring" of the circle as it were. The energy of circles seems to function by inhibiting any supernatural or magical force from passing through its bounds. Mortals are able to break it because their free will is not impeded by the energy, but supernatural beings have no free will so they can't break the energy around the wiring. This is also why random environmental debris can mess with the circles as long as it isn't "empowered" by a supernatural force or will. At one point Harry mentions that having a circle be perfectly round helps to make it more efficient, so it also begs the question if certain materials just conduct magic better like there are certain materials that conduct electricity and kinetic force better. One big thing that comes up for me is the use of objects when Harry attempts to trap a strong being. This is about creating and using images and concepts in his own mind to reinforce the spell. However there is a specific instance in Cold Days Of course this is all speculation and I'm somewhat rambling, but it's an interesting thing to ponder.