I think all the Marxist-Leninists here will agree that communist experiments always fall apart when revisionists rise up in the ranks and take over the party. Khrushchev and Deng Xiaoping brought on the collapse of attempted communist societies in the USSR and PRC, respectively.
So if revisionism is to blame for these repeated failures, when do we realize that the one-party system will always be corrupted by power seekers?
Libsocs often blame parallelism for the failures of all the previous attempts at communism.
If centralized, one-party systems always result in corruption and eventual dismantlement - it's logical to blame the failures on the lack of opposition in the totalitarian political climate.
On the other hand, a decentralized, direct democracy with a horizontal hierarchy would be nearly impossible for one power-hungry official to destroy.