When building a spellbook, there is a trade-off between flexibility and focus. Metaphorically speaking, it's a trade off between putting your eggs in more baskets, having more kinds of eggs for different kinds of predators and environments and with longer incubation periods but weaker eggshells, versus making stronger eggshells, having shorter incubation periods and fewer baskets for less varieties of eggs so they are all weak to the same predators and environments, but fewer of them.
Putting your eggs in one basket isn't always the inferior option. If you put all your eggs in one basket, then they all have your simultaneous undivided attention, and you don't have to split your resources between multiple baskets.
Aggro tends to have more focus and less flexibility, and control tends to have more flexibility and less focus. I think that this episode's emphasis on flexibility is misleading. After hearing this episode, many newer players will surely try to build their aggro decks with excessive flexibility and insufficient focus. You probably should correct that in the next episode if you haven't already. (my podcasts app lists episode 21 twice for some reason)
