Comment by latch

The library developer doesn't know what allocator will be passed, and the library user doesn't necessarily know how it will be used. They share an interface, so it will [probably] work, but it might not behave (mem/perf wise) as either expects. ArenaAllocators in particular don't play great with ArrayLists or nested ArenaAllocators.

I don't think it's a huge problem, but it's more subtle than an explicit allocator is a better because it provides more flexibility.