The command does have a practical use ( set spantree portenable disable ).
It is often used for Lag member ports to ensure that only the lag logical port (lag,0.1) can forward. The member ports will be set portenable disable so that they cannot forward independently of the lag.
Thus if there are connectivity issues and lacp protocol cannot maintain the lag, the member ports will never forward independently of the lag. The is prevents possibility of loops if protocols are failing. It also means clean failover to whatever other redundant path there is.
Hope this helps
Glyn