It would only be more efficient if user mode tasks jumped directly to each other without any kernel involvement. This is actually possible, but probably useless

It would only be more efficient if user mode tasks jumped directly to each other without any kernel involvement. This is actually possible, but probably useless except maybe some very specialized embedded applications. If you already have a privilege level change and pushing of registers on the stack, a hardware tasks switch just does useless extra work. One might think that since interrupts can switch tasks, you could set up a simple round-robin scheduler by hooking this mechanism up to the timer interrupt somehow, or have device drivers in separate tasks that become active when receiving an interrupt. But a "task state segment" on x86 is actually more like a pre-allocated stack frame, with interrupts creating a linked list of these. A task can't be reentered while it is active, and it is supposed to exit by returning to the previous task that was interrupted. Anything else requires software scheduling. The only reason to ever use this mechanism is for handling exceptions on a specially reserved stack (for example when the kernel stack overflows).