There is a lot of power/versatility in being able to run 2 instances of each block, including amps and cabs, though its not really a "make or break" thing in most situations
For example, being able to run 2 amps, and instantly/smoothly switch between them, or use both at once, does open up some creative options. Likewise, being able to have each of those amp chains have their own cab blocks, and reverbs is powerful too.
Running something like a cleaner Blackface style amp, with a 1x12 cab IR, and a spring reverb, in addition to running a Marshall type model, with a 4x12 cab, and then a studio reverb, is pretty cool.
I'm working on a kind of stereo wet/dry rig right now, and I have the rotary effect going to the stereo wet monitor, while my CLR is the dry signal. I wanted a little reverb on the dry though, just for a touch of depth, while the parallel chain with the rotary I gave a much deeper reverb. One could put the block before splitting it, and that would get the job done, but, being able to have the ability to call up a second reverb, not to mention the CPU for it, is nice, because you can simply do what you want, not work around things.
With my AX8 what I often had to do was break up parts of songs into patches, since I couldn't have like a real ambient intro with lots of effects, and then just turn those off and have CPU left for the main sections of the song. Had to change presets, while on the AXEII, you can pretty much have everything and the kitchen sink all in one patch and use just what you need as you need it.