Hello Zed; welcome to the forum.
My opinion/experience is that the ability to free you mind of any parasite thoughts (something I have a very hard time attaining) is a great asset when fighting/sparring, but not so much when learning.
That's because unless you learn from a very young age, the working principles of techniques have to be understood intellectually before you gain the ability to apply them practically to their full efficiency (yet another way to get there would be through long sessions of trial-and-error, but I'll ignore that possibility for now as being a lot slower).
Let me take an exemple.
Let's imagine that you're learning a certain technique where you unbalance your opponent by pulling him in a certain way. Your teacher shows you how to do it and systematically puts his uke
off-balance with great ease. You try to imitate him and can't seem to be quite as successful as he is, yet you don't know why. You put your hands exactly where he puts his, you feel like you're generating a lot of force, and yet you have a job making your opponent budge...
Here there are several possible scenarii.
1): Your teacher explains you that "the secret of this technique is to apply your force along an axis exactly perpendicular to the line formed by your opponent's feet. Any straying from this perfect angle, even by a few degrees, multiplies the force necessary to unbalance your opponent and makes it extra hard for you, hence your failure". That's if you're lucky enough to have an instructor who's aware of his techniques' core principles, which will save you a lot of time by sparing you the trouble of having to do the thinking yourself.
2): Your teacher is one of those people who apply the core principles of their techniques naturally, without any conscious knowledge of them. That's the case for many, particulary those who learnt at a very young age or those who reached perfect technique purely through practice (just like most English speakers would be absolutely unable to cite the grammar rules that they nonetheless apply in their speech with perfect consistency). Depending on your personality type, you'll have to either go through the time-consuming process of following in his footsteps (practicing until you get your technique right without consciously knowing why - the slow method I mentionned earlier), or work out the efficiency principles yourself.
In either case, the process should be more or less as follows:
1- You need to first understand the technique intellectually (whether it's given to you or you have to work it out yourself);
2- Then you have to practice while keeping the theoretical perfect model of the technique (principles-wise) in mind until you can indeed perform the technique "perfectly" i.e. in accordance with the principles that make it efficient;
3- Once you're able
to do the technique perfectly, you have to practice it again and again while at all times remaining extremely careful to stay true to the "perfect" (i.e. correct as far as principle are concerned) form you've learnt to perform. The objective here is to turn a mere "ability" into second nature. Being "able" to do the technique perfectly is useless until that perfect way to perform become your natural
way to do it, without the need for your intellect to keep your body in check (otherwise that perfect form is going out the window as soon as the actual fight starts, your intellect being overwhelmed. Besides the intellect is by nature too slow to be used in real combat anyway).
Only once these three steps have been successfully completed can you do away with the intellect.
If you allow your intellect to drop too early, it will hinder the process I just described by allowing your yet-not-entirely-natural technique to stray from perfection (it is crucial to stay sharp during the learning process because as someone once said quite rightly on this board, "practice doesn't make perfect, practice makes permanent. Incorrect practice leads to permanently incorrect form").
Once there, however, the intellect does indeed become a hinderance (at best it will significantly slow you down in combat, at worst it will confuse you into terrible tactical mistakes and possibly make you freeze at the wrong moment) hence the need for muga-mushin
This kind of matter is always a pain in the neck to express through written words, I hope what I typed makes sense...
Anyway, that's just my very humble opinion.
I hope it helps.