This post is another dedication to Comic-Z.
Personally, I still believe that anything truly predictable should have a counter to it. Certainly, if you know what is coming at you then you should be able to either beat/punish it or at least avoid taking damage.
Unpredictability is really quite important, though that doesn't necessarily mean random. Rather, knowing all your options and all your opponent's options at any given time, and keeping a "poker face" on.
To use an example, let's say that character (A) is unable to anti-air character (B) from the ground (heinous) and so the best option for beating/stopping jump-in abuse is to jump vertically and go air-to-air. In this situation it's necessary to act preemptively, on anticipation.
Because you're acting on anticipation, assuming you know what the opponent will do, this is where yomi and predictability comes in. If you constantly vertical jump in order to totally prevent a jump-in attack, then your opponent will easily know what to expect and take a direct counter to it, such as dashing in and anti-airing you from the ground.
If you eventually abandon your vertical jump option because it keeps getting directly countered, you may instead start using c.MK preemptively in order to keep your opponent from dashing in like they were before.
But again, if you continually only rely on your c.MK option then your opponent will again easily know what to expect and start jumping again, which will beat/punish your c.MK attempts. You can't simply rely on one option and one option only, because the opponent won't simply abandon or give up their options, they will force you to guess whether they will jump or dash. So, if you're only trying to stop one they will just use the other. But there is no one guaranteed stop-all, because even if you find an attack that covers both options, if you use that attack predictably then the opponent will just find a way to counter that too.
This scenario would be like playing Rock-Paper-Scissors and constantly throwing out only one option (like paper) before the opponent does anything, and then until the opponent has had time to see which option you're tossing out repeatedly and counters it accordingly. In other words, you keep whiffing Paper out in the open, trying to shut down Rock, when the opponent isn't relying only on Rock and is able to cut you down with Scissors.
Hence, the "poker face", where you wait until the situation actually arrives before you reveal the option you've decided to go with. Ideally you don't want it to be easy for the opponent to read exactly what you have planned, but this isn't the same as being random. Randomly selecting from a set of three logical options based on what you believe your opponent will do is entirely different from performing completely random and illogical acts. However, educated guessing based on probability, weighing risk/reward, using reaction instead of anticipation, and option selecting when applicable can all greatly reduce the necessity of simple blind picking.
Fighting games aren't really pure RPS anyway because not all options are created equal. Some options are more rewarding and less risky than others, so being able to identify what options are better on risk/reward is a good idea.
It's also important to understand advantage and disadvantage. When the opponent connects with an attack hit/block such as a jump-in you're not put at massive frame disadvantage (assuming the jump-in was deep). This limits your options for defense, and increases the aggressor's ability to do a mixup or continue pressure. Constantly using the same option for defense is the same as the above scenario of predictable yomi.
And this is commonly where the word "respect" gets tossed around in regards to fighting games. If you don't respect the fact that you're at disadvantage and don't understand or don't respect your opponent's options you often find yourself getting smashed in the face repeatedly by things like frametraps/staggers/suki. But then again, if you're too respectful you will most likely find yourself getting steamrolled by high/low/throw mixups because you're not attempting to defend yourself. So once again, it's necessary to evaluate the situation and not be too predictable in your choice of respect or lack of respect.
Of course ideally (in a dream world) you'd always choose the correct option for any given situation every time, but the correct answer to a situation isn't always the same answer as it was the last time the situation occurred (unless your opponent always chooses the wrong answer and never learns).