The idea of varying your training from time to time can be either beneficial and productive or counterproductive, depending upon how you use variety.
We all have times when we go a bit stale, get discouraged and feel that training is not producing the results it used to. At such times variety can be a blessing. The trick, of course, is to vary your exercises among productive alternatives and not among essentially useless "fancy" movements such as replacing barbell curls with seated concentration curls. And, when shuffling a program around for stimulating enthusiasm, stay away from set-rep schemes that make gains impossible, such as doing 8 sets of 10 curls, instead of your usual 3 sets of 6 reps! Such a drastic, sudden change will be "stimulating" all right. It will stimulate for the first workout; thereafter you'll enjoy training about as much as a trip to the dentist.
The essential point to remember is that varying your training does not mean losing sight of common sense.
Often, a trainee gets used to, fond of, and attached to a particular training schedule even long after he has gotten whatever good he can get from it. (Sometimes a program can and should be used again in the future, but only after a rest and change to jolt the muscles out of a familiar groove.)