My specific issue with this article is that it seems to think of practicing as one monolithic event--as though you are really just doing one big thing when you practice: getting better at what you are practicing. I think the concept of randomizing tasks is sort of getting at a good point, but it's really not all the way there. And also, you have to understand that you are, in fact, getting something extremely valuable out of the block practicing: the ability to focus on a singular task for a long period of time. That's a skill itself. You will never do well in a 3-5 hour rehearsal with a string quartet or an orchestra if this is not something you are accustomed to doing on a regular basis.
Here are some more details.
The big thing to do when I was a kid was to send your children to a practice camp like Meadowmount, where all the Juilliard School teachers spent their summers teaching kids like me. That was all block practice. 5-8 hours a day, and a lesson every day or so. That kind of block practicing went exactly as described in the article. An hour of scales, and hour of etudes, an hour on a piece of music, etc.
In my opinion, that's a mixed bag. You need to be able to pull a scale out and just lay it down. Any scale. Any time. It's a kind of a musical Fizz-Buzz problem. You just need to be able to do it even if it's not really interesting. Although, it's not like Fizz Buzz because scales are actually really hard. Etudes are a sort of micro-problem to solve. You isolate a particular bow or finger technique, and you spend some time every day working out that physical skill--the logistics of getting x fingers from point a to point b in the requisite amount of time.
Putting in a block of time on your musical piece is also important in itself because you have to commit that music to memory. Not just the notes and the finger motions involved, but all the little things like the bowings and phrasings and dynamics and stuff. The parts that take notes and turn them into "music."
The reason I think the study is flawed is that I think it's studying a strawman of an idea. No great players actually practice in the way that this article describes. There's this idea of a mindless slaving away that we do for hour after endless hour. That's not real. I mean, it's probably real for some people. But great players don't do that to begin with. And good teachers don't recommend this approach.
There are some popular methods of learning difficult things. There's a thing that people do to tackle a difficult bit of notes where you just practice slowly with a metronome and you gradually increase the tempo. Start with a tempo slow enough for you to get the notes out, no matter how slow. Then you just gradually make it faster and faster. This should work, right? No. It should not work. And it doesn't work.
--a brief interlude before I explain why that doesn't work--
What you're doing when you play a musical instrument is that you're solving physical problems--usually without the necessary tools. You are working a variety of physical challnges. The logistics of moving fingers from one place to another, you're solving acoustical problems by managing the friction between your bow and your string(s). Or if you're a wind player, you're dealing with the physical properties of your breath interacting with a vibrating reed or you're vibrating your lips when you play a horn of some kind. There are two parts of every musical problem. 1. There's the conceptual problem: "How should I be trying to execute this solution?" 2. The physical part: "How do I train my fingers, arms, hands, lips, whatever to perform this solution?"
Tempo matters when you are planning out these solutions. A slow tempo uses different physical motions, and it takes a different mindset. Slow tempos (yes, tempi, I know, but I don't feel like the word is so foreign that I need to pluralize it in Italian, so tempos) allow for larger motions and gestures. Slow tempos do not need to have efficient motions the way fast ones do.
--back to my point--
Practicing slowly will never get you to perform things quickly. You are practicing the wrong thing. If you want to learn to play things quickly, you have to practice them quickly. But in small chunk. Take 2 or 3 or 5 notes. Play them even faster than you ever intend to perform them, and do it with a metronome, so your rhythm is good. Chain them together, and gradually slow down and enlarge your motions.
I don't know the details of this study: who were the participants, where they came from, or what their experience is. But to me, this seems like a situation where the block practicers are basically dumb practicers. Because no good musicians practice that way. I think you would have positive results doing absolutely anything to break up this kind of a thoughtless, put-in-some-time-and-hope-I-get-better kind of a routine. Random exercises are probably better than that.
I can see how these results make sense. Randomization is probably a good thing for a certain type of musician. But I would argue that it's definitely not better than a thoughtful, focused, problem-solving approach applied for long periods of time.
Anyway, just thought I would throw some things out there. Cheers.