I would never discourage time in the weight room. But in my very small sample size of one 15 year old, weights have not increased velocity like we hoped they would.Yup, she's in the top 3% (IMHO) if she's throwing 59 at 13. My DD is the same and we really aren't thinking or worrying about more speed at all. I get wanting more, but you're already ahead of almost everyone.
Obviously the increases from 40 to 45 and 45 to 50 come really fast, as they grow and mechanics improve by leaps and bounds. But that's gonna slow off a ton as they have good mechanics and don't grow much more. Totally agree with the post above: our biggest improvements come after our 6-8 week total pitching break in Nov-January. We show up to team practice and the pitchers that worked hard in the off-season seem to be about the same and my DD seems faster. I tell them what we did, they just don't seem to believe it I guess. The body needs rest to come back stronger. To repair the muscle fibers. And honestly, the brain needs a break too. And perhaps the spirit.
And yes, any core and leg strength is huge. But I'm not planning on turning my DD into a gym rat just to gain a few MPH. Just too many other things in life she wants and needs to be doing. We do weights in off season and when we can during the season, but it's not a top priority.
My daughter's HS team hired a strength coach and she works with the girls every school day. DDs lifts have exploded. She's literally the strongest girl on both of her teams and it isn't even that close. But her velocity has not budged.
Honestly I would not have believed it if I didn't witness it in person. She's not going to stop lifting. But the results have been disappointing.