It's not speed or strikes. It's both. The distinction I'm making and apparently doing a poor job at is when a pitcher is learning the mechanics of a fastball arm circle, IMO it should be learned with the pitcher throwing as hard as they can, not throwing at 70, 80 or 90% so they can be accurate. When a pitcher refines their mechanics, they will throw strikes. Refining the mechanics takes time and work. Repetition and muscle memory. Other than winning a couple 10u games, I don't see what benefit a child gets learning mechanics at 80% of their max level and then having to relearn at 100% down the road. As an aside, I wonder how a player throwing 80-90% of her max to throw strikes has their change up effected where you should drop the speed by another 20-25% (another thread I guess).
As I understand it, most experienced pitchers don't throw a majority of their pitches at 100% effort. Good mechanics should be in place at all effort levels, but you will find that most pitchers are throwing their fast ball in games at (what I see commonly referred to as ) "cruising speed", which is a little less (90-95%?) than their all-out fastest. Why? Because accuracy matters!