Felix Felicis has another, more subtle interaction outside of Clash: it protects you from mill. It effectively says "whenever you would mill cards, mill one less." For example, if you get targeted by Tome Scour, you tap Felix Felicis in response to put it back on top of your library, it gets milled along with four other cards, and it goes right back into play, so you only lost four cards. Against something that mills you one card at a time, like Jace's Erasure, you become immune to the effect. That makes it both an enabler in a clash deck, and a sideboard against mill. It's obviously narrow, I agree, but it's rare. Rare cards are allowed to be narrow.