M. Potter asks why some philosophers intuitively require so much less evidence for introducing abstracta than for concrete objects. How come the requirement not to "multiply entities beyond necessity" doesn't apply to these? Without an answer taking this relaxed attitude towards positing yuppie cliques and category theoretic arrows, while being very skeptical about the Loch Ness Monster looks a bit unprincipled. Well here's a sketch of an answer.
Start with a reliability based notion of justification: we evaluate a creature's justification by thinking of it as having certain faculties i.e. mechanisms that reliably produce true beliefs (e.g. infra-red vision, smell, first order logic). We say a belief is justified when it is the result of one of these reliable mechanisms 'working as intended'. Now in order for mechanisms that produce contingent beliefs to be reliable, they will typically have to be causally sensitive to facts about the outside world - so that e.g. they tend to only produce the belief "there's a llama" in situations where there is actually a llama. In contrast, you can build a faculty that reliably produces the right results about necessary aspects of the world, without using any such external input. And if there are necessary truths such as: whenever there are yuppies behaving in such and such a way there's a clique of yuppies, you can build in a reliable mechanism that makes this transition immediately, without requiring any further input from the environment. So it's not surprising that the reliable belief forming mechanisms we humans have should require less justification for introducing necessary abstracta, or ordinary objects whose existence is necessitated by already known facts about other objects vs. for introducing concrete objects (like the Loch Ness Monster) which lack either of these properties.
Now obviously, what I just said won't convince anyone who has some *other* other reason for rejecting abstract objects, and ordinary objects, to believe in them. But it does provide a unifying explanation, and hence (I think) a way for those who a) have the intuition that introducing abstract objects needs less justification and b) are inclined to take this intuition at face value to defeat Potter's challenge that their intuitions about justification are unprincipled. Quite to the contrary, this distinction falls out of a reliable-mechanisms theory of justification almost immediately!