Air spindles aren't as stout as high speed machine. Any adapter for that matter.
They do however serve their purpose; To add High speed machining capabilities to low speed machines.
Most high speed tooling is also high relief, and most high speed processes are low load processes, so you don't need as much ridgitity any way.
Another good call for them, is it is usually less expensive to add an air spindle to a standard machine than it is to buy or upgrade to a high speed machine.
High speed machines also lack low speed torque neccassary for some operations, so this would allow you to basically have 2 machines in one.