Who or Whom? which would be correct?

[Mr Downtown]

Ammo wrote:
Is this rule simply a convention, or does serve a useful function?

Well, all of English usage is simply a convention. In this case, it's a bit of redundancy to help clarify the meaning of the sentence, which can be interpreted two different ways. The same rule is normally used to distinguish whether that or which is proper.

If the clause who I consider friends can be omitted without changing the writer's meaning, it's a parenthetical phrase that simply adds a bit of side information. Thus it should "stand alone," and whom becomes the object of the verb consider.

If the clause is necessary to the sentence's meaning—I only visit friends among the residents—then it modifies (enhances the meaning of) the sentence's main structure. Using the ordinary nominative case who clarifies this for careful English speakers who still distinguish the two forms.

^^^see what I did there? It's a clause necessary to the meaning of the sentence, so no comma and who.