Some of this surely stems from Rust not originally supporting allocators at the standard API level until later. You can manually create your own, and create/modify your own containers, but until GlobalAlloc came around it sometimes involved doing things like passing through Arc<Allocator> through to things if you wanted multiple things to use a single one.

Sometimes using a global one isn't the best thing (it's often a good idea to specialise base on allocation size, reuse and lifetimes), but I've used them quite a bit in C++ over the past 16 years doing HPC for graphics, rendering and simulation, so calling them only useful for "toy problems" likely shows you just haven't found a need for them in what you've been doing.