The current modeler paradigm is largely built around making full rigs inside one preset, and some modelers are powerful enough to handle complex kitchen sink presets. But many are not, and that's when global blocks can become relevant as you might want to swap the array of fx around your global amp/cab blocks for example.
I'd add to this that even if you could build the biggest kitchen sink preset in the world, you probably just don't want to.
I remember coming back to one of my Floor presets after a while of not using it and I was all like "WTF did I do there and why?!?" - and it's been pretty much impossible to reverse engineer things (not necessarily the "how"s but the "why"s).
So spreading things over multiple presets often is a lot easier.
On my current GT-1000 based setup I'm doing just that - and while I'm not doing much more in one bank of 5 presets than with the kitchen sink stuff I used to use with the HX Floor, it's quite easier to manage.
Let alone, it frees up footswitch capacity. Spread the load over 2 presets = have twice as much switches available for manipulation (well, not exactly, but you get the idea).
The main issue I see with global block is how to handle assignments to snahpshots/scenese/controllers/whatever if they are not constant throughout the presets.
That's a pretty valid issue.
For a start (there might be better solutions and not everything might be covered), values programmed per snapshot could be "offsets" of the global value. This is kinda like how modulations work on synths.
Example: Let's say some parameter X has value 100 in snapshot 1 and you wanted it to be at 70 in snapshot 2, you'd program an offset of 30% ("program" not being the proper word as you would just have a simple offset knob/slider, possibly also showing you the global value, just so you knew how far you could go).
Sure, that won't work if you were sure about certain values for certain snapshots - but in that case, you'd not lose anything compared to the current situation but just not be able to use that block in a global fashion.
Another approach could be the ability to split the grid and dynamically link one part to the same area in another preset.
There are more variants.
This would only work on certain units. But hey, why not? I mean, in the proposal I posted in the Stadium thread, *anything* could be globalized, so what you're suggesting would as well be possible.