I totally agree with this!!
As you say ( @Blue Dragon ) the 2x combo option works great for a home and rehearsal space rig, and you can always join them together when needed.
But as @Gravedigger Dav points out, 700W of Rumble is a hell of a lot. As much as we can all dream of the "Rumble Wall Of Doom", I can't think any of us could logically name 1 situation in which we could actually use (never mind need) 1000W of Rumble! It is good for fun once in a while to say "Hey, look listen to what I can do!", but it is not useful in a band situation (unless you are trying to replace Lemmy).
My "big" rig before the Rumble was a 1200W setup, which I could actually increase to 2000W if I wanted, but even when backed up with a 10.000W PA (sometimes even bigger), not once could I turn the rig up above 50% without it completely drowwing out the stage and the first 20 rows of people. I did do it on ocasions during rehearsal (or even a few times at soundcheck) to make the point of "I can make it unbearable if you don't turn down!", but it was disturbing!
I would say that 2x combos, plus maybe 1x extension cab, could be very useful, but more for practicality than volume.
I have yet to find myself in a situation where I need to max the 500C out, and until then, I can't even warrant buying a single extension cab. I would rather spend the money on more bass(es). Don't get me wrong, I love the Rumble and all its tones, but I get all that with just the 500C, what more do I need?
(note: I do have a couple of PA systems, and in 90% of the live situations I run through PA, YMMV obviously)