If people have to draft the lessons too I certainly think it is. You don't give up deck space, but cube real estate. I don't want to free a slot in my golgari hybrid section for Pest Summoning. It's just not worth it without learn-cards. You need to draft mechanic A and B, and I highly doubt we will get enough of either in one single set for most cube sizes and power levels.
While it's possible that cube real estate could be a problem when implementing Learn+Lessons, that isn't necessarily going to be the case. If the floor of the Learn cards is good enough, adding lessons to the cube becomes a free roll.
You have to remember that any card that doesn't make it into a player's mainboard is effectively a wasted slot, at least for that draft. Since players draft 45 card pools, but a deck only requires 40 cards, there is always going to be at least 5 cards "spoiled" per player per draft. This calculation also assumes that every land in the deck was drafted as a part of the pool- something which is very unlikely in most cubes. Now obviously some number of unplayed cards are going to be off-color as the result of the end of the pack, pivots, or failed build-arounds. However, there are still often a substantial number of real playables that get cut from a player's final deck. Using Cube space on cards which can effect the game from sideboards is a strategy to reduce the number of cards that outside the game.
It's also important to consider the fact that learn is far less parasitic than other mechanics that care about specific things such as Energy, Mutate, or Tribal themes. One does not need a single lesson in their cube for a learn card to be worthwhile, provided that the rummage trigger is something that is desirable for their deck. I would consider playing the Professor of Symbology as is- it has a lot of overlap with a lot of different decks in my cube! Unless the professor is the best Learn card in the set (which I doubt it is), there's going to be some number of playable Learn cards. At that point, running Lessons is a great means to boost the power level of cards we are already playing.
I could easily see a cube with only 3 or so Learn cards and a single Lesson.
