True, but GC gives those benefits automatically, compared to a naive program doing e.g. RC-based memory management.

And there is of course the question of safety; should you release an arena too early, you may have introduced a bug. Worse: it might not crash immediately.

There is actually some work for doing arena management automatically, called region inference: http://www.mlton.org/Regions

But the way I see it, it's just a way to make memory management even more efficient; it's not about precise release of resources, and indeed not all programs can be expressed so that releases can happen only in batches of an arena (assuming those arenas themselves aren't dynamically managed, which certainly is a valid strategy as well, but manual).