If you want to maintain the rule you should better invest sufficient continuous energy to keep the rule adherence at a level where it is useful instead of letting it slip and then invest significant energy into getting it back.

I agree with this.

I also agree with you about the importance of making children feel significant. Hand in hand with children expending more brain power on their parents then the parents spend on them, I have a much larger world of which they are just a part, whereas until they are older, I am a much bigger part of their reality. I try to keep this asymmetry in mind and try to be more present for them. (This is one of my biggest challenges as a parent.)

Again you infer too much from my isolated remark that it be risky.

Actually, what I was responded to was more central in your post:

If the children notice this they may assume that you either condone, accept, bear or ignore it. None of these has positive effects.

My point was that I think it has mostly positively effects, because it teaches them the truth. If the parent didn't interfere with the undisciplined behavior (for example, if they are too tired or preoccupied or conflicted) then it is probably the case the parents condones/accepts/bears that behavior at that moment.

A long time ago I agreed that we establish the same rules and back each other on decisions made by the other. But often enough I find myself in a situation where I have to back her - but don't know to what end.

We struggle with this also. For example, with keeping a schedule and with routines. It completely doesn't work unless both parents maintain it, but in our case it is more important to one parent than the other.