[davidgtl]

The simplest explanation I've heard is that hyperthreading is like using two hands to keep the mouth always full(whereas with only one sometimes it remains idle). Once you can keep it always full, there's no use adding more hands.

[marcan_42]

POWER has 8-way SMT. More than 2-way SMT is definitely useful in some cases, but it does not exist on any x86 chip, so it doesn't matter if you try to use more threads in software. The hardware can only handle 2 per core.