Ask Lemmygrad

You are correct. I can't speak much for Mao's writings, but Marx and Engels consider them useless and even dangerous to the revolution, based on their own experiences at the time.

As the name suggests, they are proletarians. However, they lack any sort of class consciousness, and for various reasons, they argue that it would be extremely hard to reform them. Furthermore, they are very susceptible to becoming tools for reactionaries. Marx and Engels had observed this first and third hand in many occasions.

They've kept the term very vague, but they do define it somewhat sometimes

The lumpenproletariat is passive decaying matter of the lowest layers of the old society, is here and there thrust into the [progressive] movement by a proletarian revolution; [however,] in accordance with its whole way of life, it is more likely to sell out to reactionary intrigues.

The Communist Manifesto, Marx & Engels

Alongside ruined roués with questionable means of support and of dubious origin, degenerate and adventurous scions of the bourgeoisie, there were vagabonds, discharged soldiers, discharged convicts, runaway galley slaves, swindlers, charlatans, lazzaroni, pickpockets, tricksters, gamblers, procurers, brothel keepers, porters, literati, organ grinders, rag-pickers, knife-grinders, tinkers, beggars; in short, the entirely undefined, disintegrating mass, thrown hither and yon, which the French call la bohème.

The Eighteenth Brumaire of Louis Napoleon, Marx

So these are the main groups of lumpenproletariat, and why they can't be trusted:

1. The criminals: They are self-serving and greedy to the extreme, and their activities usually involve exploiting other proletarians through threats, violence or deception. If placed inside a movement, they would have no qualms about betraying it by acting as spies, informants or sabboteurs.

2. Prostitutes, drunkards, beggars, addicts: Poverty or addiction has turned these extremely desperate. Their desperation can be exploited and made to also turn into informants.

3. On-and-off workers: Meaning workers who work here and there at various jobs without having a steady salary. Very common thing in the 19th century, where people would gather at a particular street, and then bosses would come by, shout they need these many workers for this work and this pay, and then these people would rush to sign up. Extremely exploited, as they would outcompete each other over who will do the most work for the least pay. Due to their desperation, they are often utilized as strikebreakers.

4. Other people: Who for whatever reason will always be loyal to reactionary forces. In today's terms, that would mean for example an anti-gay Christian conservative in the US, who is dirt poor, but will always support someone like Lindsay Graham, because of his devotion to being anti-gay. If I recall correctly, Marx in a letter referred to retired German soldiers who are conditioned into protecting the upper classes at all costs.

Now, this is a controversial issue, and it's great that you stumbled upon it because, one has to understand that:

a) Marx and Engels lived 200 years ago. Some of their ideas conform more to that time period than to today.

b) You don't have to agree on everything with Marx, as long as you agree with the basics.

Mao, on the other hand, even if he did agree with Marx mostly, he believed it's possible to reform lumpenproletariat, or at least manipulate them, into serving the purposes of the revolution without betraying it.

The important bit:

As you yourself experienced, discussions on the lumpenproletariat usually devolve into arguments about whether or not one should empathize with the lumpenproletariat. This makes people tend to forget whether they should heed Marx's and Engels' warning. I believe 20th century history is rife with examples of why the Lumpenproletariat should not be trusted. Notably:

• All fascists had a base core of supporters from the lumpenproletariat, who became the most devoted foot soldiers for those regimes.

• During the cold war, the most anti-communist segment of the western populace was the lumpenproletariat. To the extend where criminal gangs openly colluded with government organizations like the CIA to destroy socialist movements (e.g. The Mafia collaborating with the CIA in Italy and the US).

• The lumpenproletariat was used extensively to infiltrate leftist parties and break them apart from within. They've also been used as the primary strikebreaking force.