Do you really think the bourgeois would allow the slaves to revolt? Leftists must be armed because the example of an alternative to soul crushing capitalism will give their slaves hope.

'Under no pretext should arms and ammunition be surrendered; any attempt to disarm the workers must be frustrated, by force if necessary' - Karl Marx

You should think a little about your automatic reation on the word "junta" with "they are no doubt brutal" and where it comes from. Those particular juntas enjoy the widespread support and acceptation in society and are much less brutal to it than your average western (not to mention comprador) "democracy".