A lot of people use "it" as written b/c it works. But a lot also use what they LEARNED from "it" and are still training using the same big principles behind the 5x5, but their training does not look exactly like the 5x5 as written. Read this:
http://www.elitefitness.com/forum/showpost.php?p=6622374&postcount=6
Before "it" showed up, a lot of guys shotgunned multitudinous exercises and set/rep schemes at their bodies and hoped for the best week-to-week. FLEX light, I imagine. 10 sets of different types of curls on Mondays. 8 sets of pulldowns with various odd-shaped handles. etc. etc. And guaged progress based on soreness, mental excitedness of each workout, pump, whether or not their workout was deemed hardcore by the masses, etc. Lots of solid indicators of progress like the above.