There are very few scenarios where switching your library for your hand are valuable
You'll often see these in use at MagicCons in the Command Zone, and for good reason, they're much smaller and easier to organize than a full backpack
Both halves matter against a deck like Dimir, but the fact that one half will matter against most decks means this is likely maindeckable
In the actual lists, however, he gives you ridiculous amounts of value by being able to flash in stuff at opportune moments, enabling you to get around opposing interaction and set up your wins for a strong finish
Beyond Boosters are mainly for collecting, especially for fans of a specific universe
There's no real number, but control decks generally run fewer creatures than other archetypes