A random phase can keep people from falling asleep. so it's a good idea to include them. however, their difficulty can go all over the place, because, well, they are random. They tend to range from joke to moderately challenging (for the easy ones) and from hard to unfair for the toughest ones.
There is a better way.
Test the phase yourself with user specified seeds. if the seed is too easy, or too hard for you, try a new one. Once you have collected 50 or 100 seeds of decent difficulty, randomly select a seed to start the pattern off with.
You now have something that cannot be memorized, yet won't give the player a cheap shot, since cheap shots in random patterns are just not fun. (save the cheap shots for static and streaming patterns.) It plays just like a standard random phase, but doesn't ever screw the player over or give a free ride.
I Wanna Clear the Easy Miku is pretty reasonable with it's randomness (if you pay attention you can deal with it all pretty easily). But for the second one, the randomness puts the difficulty all OVER the place, and it suffers for it. While the random barrages that expand from Miku always seem fair, the volcano and the second negi wave are all over the place. And the small box section can get really rude at times.
Now if you find 50 seeds without discarding any for being too easy or too hard, then you have designed a good random phase, and probably don't need the workaround.