Agree with what you've said.
Users are not able to create index on the global temporary tables. It looks like they can but they have no effect. Only the create table statement is used for the underlying temp table.
So
create global temporary table xxx ( i int primary key )
will create an index that can be used.
We're only aiming to use where we would have used @table variables and more usefully in C# Connection Pools.
Note: I renamed a global temporary table and soon afterwards the server crashed - not sure if the 2 things are connected - so will try this again.