I think it's very useful, and a totally realistic comparison. Now I'm on Go, I can see it would be cheaper on Agile without me doing anything different to how I do it now. So if I switch to Agile I'll at least make a saving, before I even do things to move use out side of the peaks. Thus a win. Or if I were on Agile, doing my best and Go were cheaper I could do nothing different (or since all day is flat rate, simplify and makes things more convenient) and again save money.
It's a realistic comparison of how a person who uses their leccy would have fared if they were on another tariff. Sure, if you adjust your behaviour then that would change things, but not everyone can, or is willing to do that. Take me as an example. There's no way I'd be able to shift our evening mealtimes out of the 4-7pm peak, not a chance. And while I could change when I charge the car, I'm barely doing any miles at the moment. So the reality is most of my usage is in the "normal period" of Go, and would be in the normal period of Agile... thus it allows me to compare how that would have done.
So if you're prepared to deep dive into the thinking of how you use electricity, usage patterns and analyse what it says, its a very useful comparison indeed.