There are a few out there like Vern Gambetta and some other well know strength coaches that claim this idea is complete nonsense. But current research shows they may be wrong.
The below is from a PowerPoint that I use to teach a college level class which covers the different training principles.
The muscle confusion principle
The research says: A study conducted by researchers at the Tempe campus of Arizona State University compared two groups of trained subjects, one using an undulating periodization program—a fancy term for muscle confusion—and the other with a linear periodization program. In the undulating program, the acute variables were changed with every workout, whereas in the linear program, they were changed every few weeks. After 12 weeks, people following the undulating periodization program increased their bench press and leg press strength by 100 percent more than those following the linear program.
Researchers from Brazil saw even more drastic differences when they had trained subjects follow one of three programs: an undulating periodization program, a linear periodization program, or a consistent non-periodized program of 8-10 reps per set. The training program consisted of a two-day split with 3-4 total training days per week.
After 12 weeks, the group following the muscle confusion program increased its bench press by about 60 pounds, almost 200 percent more than both the linear periodization program and the non-periodized program. The undulating program also increased the subjects' strength on the leg press by a staggering 275 pounds, 400 percent more than the non-periodized program and more than 300 percent more than the linear program.
This principle can apply to muscle growth as well as strength. Researchers from the Federal University of Rio De Janeiro had a group of untrained men follow a linear periodization program of two sets of 12-15 reps per exercise for four weeks, then three sets of 8-10 reps per exercises for four weeks, and finally four sets of 3-5 reps per exercise for the final four weeks.
Meanwhile, a group used those same set and rep ranges but cycled them each time they trained. The results: The group following the undulating plan increased its triceps size by about five percent, while the linear group saw no such increase. The undulating group also increased its biceps size by 10 percent—twice that of the linear group.
Yes, Louie Simmons of Westside Barbell also used this idea in training. It has been done successfully by some of the top pro level bodybuilders for many years. Joe Weider observed this method back in 1950. Any time you can change the plane of movement, modality, reps, volume and TUT you are forcing the body out of homeostasis, thus forcing it to adapt to a new stimulus.
Rhea MR, Ball SD, Phillips WT, Burkett LN.A comparison of linear and daily undulating periodized programs with equated volume and intensity for strength. J Strength Cond Res. 2002 May;16(2):250-5.
Miranda F, Simão R, Rhea M, Bunker D, Prestes J, Leite RD, Miranda H, de Salles BF, Novaes J. Effects of linear vs. daily undulatory periodized resistance training on maximal and submaximal strength gains. J Strength Cond Res. 2011 Jul;25(7):1824-30.5.