I agree that SAGE is an approximation to Nash. When you play online, you can just use a Nash equilibrium chart, and so I don't see a reason to use SAGE instead of Nash.

If a new player is about to play Chris Ferguson in heads-up live play, and you have to tell him something, SAGE may be simpler to explain or easier to remember than the Nash equilibrium, but I have never used it. Since I have played so much while looking at the Nash chart (in a book by my side, or on my desktop wallpaper, or bookmarked in a web browser, or on a T-shirt) I remember the pushing values quite well without using approximations. For example, you can push any two cards 9 or higher for 20 big blinds, and any two suited 6 or higher except J6s. 76o is a push for just over 10 big blinds, while 65o is not a Nash push for 3 big blinds.

It is a little less valuable to remember the Nash calling ranges, since those are not used in Golden Rule shoves, and many opponents are pushing too tightly. However, I also remember particular references such as that Q9o, Q7s, K5o, and K2s are marginal Nash calls for 10 bb.

I recommend watching the semifinal of the 2009 National Heads-Up Championship between Farha and Seed to see that knowing the proper push/fold strategy matters a lot in practice. You can see this on Hulu:

Part 1 Part 2. Huck Seed had done his homework, and Sam Farha had not. When it became clear that Farha was not following the Nash calling strategy, Huck Seed made some small deviations from the Nash pushing strategy, and had a huge advantage.