Originally Posted by Rafiq
Marxism is omnipotent...
Upon first read, I honestly read "impotent"... 
But I digress.
To be honest, this is a pretty weird thread. It's kind of backwards, in the sense that you're asking for proof against something rather than supporting evidence. Marxism is the premise, so it follows it's the conclusion. Similar to the ways theists argue for proofs against theism based on their pre-conceived premises.
As a result, I think there's more potential in this thread for Marxism self-criticism, rather than ideologically divergent criticisms given the framing.
I don't think I can thus contribute much more to this thread, though, I would be interested to ask, when you mean "valid" arguments, do you mean in a polemical sense?
[formerly Cthenthar]
Revolutionaries don't spend all day on a messageboard. Action is realisation of the polemic.
"When the lie returns to the mouth of the powerful, our voice of fire will speak again." - quote EZLN
“Development develops inequality.” ― Eduardo Galeano, Open Veins of Latin America: Five Centuries of the Pillage of a Continent