Carbon fiber frames are worth the premium if you're a competitive rider or seriously weight-conscious. They're the lightest option available, and engineers can tune the carbon layup to be stiff where you need power transfer (like the bottom bracket) while still absorbing trail chatter for a smoother ride. That kind of tunability is something aluminum simply can't match.
Don't let old myths about carbon being fragile scare you off—modern carbon mountain bike frames are thoroughly tested and extremely durable for normal riding. The real vulnerability is sharp impacts from crashes or rock strikes, which can cause damage that requires inspection.
Carbon makes the most sense for XC racing, trail riding, and enduro where every gram matters and you want that tuned ride feel. If you're a casual rider or on a tight budget, aluminum delivers excellent performance at a much lower price. But if you can afford it and want top-tier performance, carbon delivers.