A statue of Engels and/or commemoration/discussion of him (and Marx) as part of this city is not the same as a celebration of communism, in my view.
Communism is pretty much proven to be a busted flush in terms of being a successful method of running a country/region/economy. Whether that is down to the inherent corruption of humans in power (especially in that sort of system) or it just being hugely flawed, we can probably all agree that it is a complete failure.
That's not to say that capitalism has been proven to be a particularly good system - with the death, poverty and misery it has also visited on millions. However, it is almost certainly the best of a very small bad bunch of alternatives that we have.
But Engels and Marx were a) not involved in the implementation of their theories and the poor/selfish/murderous decisions that were taken when doing this. And b) were two men who, regardless of their personal circumstances, were motivated by a desire to assist a huge section of society that, at the time, were subject to conditions and exploitation that were a stain on society and little more than a disgrace.
A quick bit of research of the conditions that people were living in in areas of Manchester like Angel Meadow (an area that they studied) should be enough to demonstrate to even the most rabid individual that it was probably out of concern for people that they developed their theories. And whilst communism has gone on to be a complete disaster, sympathy for the down trodden and the exploited is still, even now, something that is considered a good thing. As that was their motivation - even though they might have actually been flawed, misguided or downright wrong, I don't see any reason to not recognise their place in this city's history. You could also argue that their mere existence inspired advances in the circumstances of ordinary people, even if that was not what they wanted, as there is little doubt that concessions and changes agreed to by the ruling classes were agreed to in the face, at best, recognition that people writing about the unfairness of conditions had a point, or, at worst, fear of revolution.
If anything, the impact of their writings on the modern history of the world is, to put it mildly, noteworthy, at least. To ignore that would be bizarre.
