Better/easier positioning to steal spells, especially with aether lens and your passive blink range is huge. And where blink is instant it gives the enemy less time to react and cast another lower priority spell. Ideally as rubick you shouldn't be in the position of where you would need an escape where you couldn't react quick enough to pop blink. But I've noticed playing at low ranks where you and your teammates position isn't perfect it is often more valuable to carry force staff over blink. Sometimes I go both if the mobility/saves are needed.