The problem I have, is you have experienced it with one setup, I assume, since you still have 5970's. Reviews actually compare them side by side, so they can experience the difference.
I would think that someone with quadfire would still recommend going 2-way SLI/CF if they can get the same performance. It just simplifies things a bit.
The 5970's was not around 6 years ago so its not my first multi gpu setup, i had 2 more before then.
I also have 3 way 7950.
2 way has never been enough yet for me.
Its simple, quadfire and TriFire has been fantastic for me, i have also seen it being a nightmare for others, there is no guarantee how its going to be for another person so i don't encourage it but neither do i discourage it, if they want it then they can have it.
I have seen games go from a stutter fest even with Vsync to smooth as butter with a new driver, shite happens even on single gpus but people do not give up and go console because of it, its all about what matters most to the individual which some people seem to forget.
I state the benefits and the possible negatives but at the end of the day its up to them.
It works for me the majority of the time and that all that matters to me, at least i have the option of more power most of the time rather than not having it at all and needing it.