well I wouldn't say that mystical is a good sensation if you are chasing it, and it is certainly not the goal of a meditation practice, but if feeling it helps you deepen your understanding of what people mean when they say "mystical" then it's certainly useful. Moar understanding = moar good. That being said, chasing it will get you nowhere in the long run, just as chasing it before you've experienced it will make it harder to get there through meditation. To the next point, I wouldn't say that bliss states are any more or less "meaningless" than introspection states, meaning is derived super subjectively obviously, and if the "meaning" you derive is that any given moment can contain bliss that is accessible by concentration, that is certainly not a "meaningless" observation, especially if your bliss extends to the world around you.

One does not need to meditate for 10 days straight to dip into these experiences, but it certainly helps if you want todive into them. Quantity has a quality all of its own, and I don't think I'm alone in saying that the meditation you do on retreat has a certain deeper quality to it at many times. Of course there are no hard and fast rules, but I'd be surprised if you didn't feel something similiar after a retreat. In short, don't knock it till ya try it.