Ah OK. It seems you want to discuss node centralization after discussing mining centralization. I want to discuss them together, for this reason: it seems to me that the principal argument against restoring the op_return limit is that some amount of increased miner centralization pressure is foreseeable if spammers relay their spam through private mempools instead, and this centralization pressure can be reduced by relaying spam through the public network instead. But I think the phrase "the cure is worse than the disease" applies here. Specifically, the amount of centralization pressure such activity is likely to produce is small by comparison with the even worse amount of node centralization that is foreseeable if spam is relayed by default in the most popular node software. Therefore I think the two forms of centralization pressure deserve to be compared with one another, and thus discussed together.
To that end, you asked what is the purpose of properly assigning blame. It is in response to an argument against filters that seems popular to me: that they are bad because they force spammers to use private mempools. E.g. see this letter from a group of bitcoin core devs: "Knowingly refusing to relay [spam]...forces users into alternate communication channels." 
This argument is false because it commits the false dichotomy fallacy, by asserting that a spammer discouraged from doing bad action A is thereby forced to do bad action B. Thus, the argument blames the discourager for doing a good thing (trying to stop bad action A) when the only one blameworthy is the spammer, since he is the only one who did anything bad.
The fallacy is captured by a famous limmerick:
There was a young man from Darjeeling
Who got on a bus bound for Ealing
It said at the door, "Don't spit on the floor"
So he stood up and spat on the ceiling
The position that Core's former anti-spam mempool policy forced some spammers to use private mempools, and thus the policy should be removed, is equivalent to arguing that the anti-spitting-on-the-floor policy forced the man in the limmerick to spit on the ceiling, and thus the anti-spitting-on-the-floor policy should be removed.
On the contrary, if there is good evidence that the policy helps prevent spam that would otherwise harm the network through node centralization, then I think it should stay and continue doing that, unless the amount of mining centralization foreseeable is worse.
Bitcoin Core
Bitcoin Core development and transaction relay policy
Bitcoin Core development and transaction relay policy
