I cannot prove this, but I think this is true.
I don't believe that sarcoplasmic hypertrophy is "useless" or "non-functional" hypertrophy.
First, what is hypertrophy, and sarcoplasmic hypertrophy? Look here first. Short version? Sarcoplasmic hypertrophy is an increase in the "fluid" mass of your muscles, rather than an increase in the size of the muscle fibers themselves.
Why do I think this? My reasoning is simple - if it's useless, why does your body do it?
Think of it this way - your body generally has a good reason to act the way it does. Sometimes it does some entirely non-helpful things, like have allergic reactions or have your immune system overcompensate. But even then it's triggered by something, and the reaction makes sense even if it's not helpful. For example, muscle costs more energy to maintain, fat less. Therefore the body holds on to fat easily and sheds muscle mass easily. Keep doing the same motion over and over? Your body adjusts your posture to that position and becomes more and more efficient at using energy to do that motion - which is why 1 mile is hard to run de-trained but ultramarathon runners crank out 26.2 miles like it was a warmup. Lift a heavy weight? You adapt enough to lift that weight and a little bit more, but not more than that.
So, if what your body does is generally useful, and it's usually efficient. So why would it respond to weight training with non-functional muscle mass?
That muscle mass is metabolically costly to create.
The muscle mass is costly to maintain.
So, again, why would it do this if it was useless?
My belief is that this hypertrophy isn't useless; it's adjust adapted to how you train. This hypertrophy must serve a purpose - either as preparation for increased muscle fiber size, or increased fuel storage to do more reps, better leverage (which makes higher reps easier), etc. It may not be as useful for some purposes (such as explosive lifting, jumping, etc.) and may be contrary to your goals (extra size equals extra weight, which is bad in weight class sports). But it's not useless. I don't believe your body would spend so much energy creating and maintaining muscle of any kind if it was useless.
This muscle, by the way, is probably part of the origin of the "big bulk muscles" versus "lean, toned muscle" myth - it's easier to get sarcoplasmic hypertrophy with higher reps than with low reps or long, low-tension exercises. So people training for strength or endurance don't get so much of it, creating an association between the idea that big muscles aren't strong ones, or that weights always create that kind of muscle.
For more on this subject, check out this prior post: The Myth of Non-Functional Hypertrophy, where I discuss an article by Kelly Baggett.
2 hours ago