1) Are the two interchangable? Or can you at least default for a lowerer TN mod?

2) Exactly what uses have you seen for Magic Background?
(I know magic types need it and all that and I know that they have it easier than deckers when it comes to knowledge skills...)

3) Other than spell design, can Magic Background be used like the old Magic Theory?

1. Spell Design is used to design a spell, but you can default to Sorcery or Magic Backgroud with a +2 modifier to do the same thing. This rule basically castrates the main point of the Magic Background skill to begin with since the implication in the core rulebook was that it was the defacto magic know-how skill. (Not surprisingly, none of the archetypes have anything like Spell Design.)

2. Not many that weren't covered better by other Knowledge Skills. It's basically been castrated into a drummed-down "general" knowledge skill for magic, making most target numbers using it pretty high since it's not specialized. I usually don't even bother with it anymore for my magicians, preferring instead to specialize their knowledge and default from there (for the same end result in most cases).

3. Not as far as I know. It has no real use to my knowledge.

1. Yes, you can default to Magic Background to design a spell. Magic Background *is* the de-facto Magic knowledge skill; when the GM can't think of anything more appropriate, Magic Background is the best knowledge skill for anything magical.

2. Quite a few. Knowledge skills frequently overlap, so the more generalized the knowledge, oftimes the better off you are. I, personally, have had to force certain compartmentalizations onto BBB knowledge skills-- "Safe House Locations", for one, needs to be specialized by city. Magic Background gives you the widest array of knowledge on the topic of magic.

3. Kinda sorta maybe. I use it as such-- it's a general knowledge skill about all things magical.