Because you might don't want to update everything every time you touch a parameter - without an explicit confirmation, even if you used a global block. Like the snapshot reacll/discard preference, same situation.
Well, in this case I think I have to agree with molul (unless I'm misunderstanding your point), global blocks are global blocks. It should however be possible to very quickly "un-globalize" a block.
And as far as snapshots vs. global blocks and the implications coming along with that issue are concerned, as I wrote in the other thread, I still think that at least for a start, it should be "blocks can either be global or snapshot controlled" (the only exception being the on/off status, which you obviously want to be able to control via snapshots in many, possibly most, cases).
Fwiw, this would also cover any MIDI controller assignments. Once a global block is controlled from an external MIDI controller, these assignments would be valid globally, too. In fact, it's the foremost thing I'd make intense use of, by mapping all the most important parameters I'd want to access globally to an external knob box (something that I already did and drooled over like a little child over a birthday present, just that I had to cancel things due to some shortcomings already explained).
Needless to say: Whenever you decide to turn a snapshot controlled block into a global one, there should be a clear warning: "Setting the block to global mode will discard all snapshot assignments!"
I still think that introducing offsets for snapshots would be a nice idea - but it might not be possible too easily with the general software framework and there's certainly chances for confusion to emerge.