I like your quote. It reminds me of a buddy of mine who says I am a hypocrite for buying local fruits and vegetable, because I buy toilet paper made by a Koch-owned company.

For some people, unless you live in a cave and refuse all modern technology, you are not allowed to have any moral authority.

I think it's more of an appeal for validation for your [more] consistently unprincipled friend's part.

They've decided that the effort is futile, so why can't you?

Or, maybe you just got your quote wrong?

https://en.wikipedia.org/wiki/Perfect_is_the_enemy_of_good

I think that depends on the reading of "inconsistently principled". If the principles are inconsistent, then I tentatively think your equality holds. If the principles are consistent, and the stridency in application is inconsistent, then I don't think they are equal.

With the second reading, a tangential concern is what drives the inconsistency. If it is purely stochastic, then I think the above saying holds ("better to be..."). If it is manipulated by unprincipled agents, then it's much more questionable.

Keeping it intellectual, these arguments get put in front of the US supreme court all the time. Under "uses and abuses" of power, type cases. Disregarding their fundamental import has important consequences (at least if you believe wikipedia). See, eg

https://en.wikipedia.org/wiki/Equal_Protection_Clause

There's also a long history in cognitive science on social trust and perceived consistency (anti-hypocrisy).

https://en.wikipedia.org/wiki/Cognitive_dissonance

So that is why people normally vet these issues outsied of the context of SCOTUS, even on social issues. These are valid ideas irespective of people's views on any subject, IMHO.