I've just realized the issue I have with the whole concept, or at least it's finally concrete in my head now. Here's the issue. I create a block, say I describe it as a kinetic buffering field, so that it slows attacks down. However let's say I decide to surround my enemies with it. I'm preventing one type of action so it can apply to multiple targets and to be safe I'll even spend 2 shifts to make it zone wide. It still prevents them from attacking me and my allies, but now it's a offensive (or detrimental) block. I can now throw all of my foci and specializations into offense and still have no issue defending. For that matter I can still veil, maneuver or anything else provided I target an enemy and not an ally. I think that's the part where this gets hazy.
And then that one guy that you hadn't noticed turns up behind you and shoots you in the back.
Flippancy aside, I think this is an important point. The offensive blocks you're using rely on knowing the enemy's there and being able to do things to him. Most of the time, you're fine- but there are situations where it won't work and, if I was GMing a game with someone exploiting this, word would start to get round the supernatural community about how he operates- and his enemies would start using traps and ambushes a lot more.
