The point of “living your vocation” (in this case, that of marriage) is to grow in virtue. If physical self-giving everyday will help the couple grow in virtue (generosity, selflessness, patience, humility), then it would be a good thing. I can think of many reasons, some already stated by the OP and others, why frequent marital contact would aid the couple’s growth in virtue. If the couple is at a point when mutually agreed upon “fasting” from the joy of intercourse would help the couple grow in virtue (material detachment, mortification, humility). There is of course a point also at which frequent indulgence in the marital act may distract the couple from the pursuit of virtue. At this point, the couple would be wise to step back and reassess. There is also a point at which abstinence would distract the couple from growth in virtue, at which point the couple would —wait for it

----- be wise to step back and reassess.
The OP has a point that it should not *simply *be the “mood” of either one or both that completely dictates the couple’s physical intimate life. The sacrament of marriage is too precious and yes, fragile, to leave such an important part of it to the whims of “mood”. There are many other things that must be considered. I think really we all agree with that. There are probably some couples that are better and more intuitive at aligning their intimate relationship to the proper practice of virtue, and so don’t realize the work that it may take another couple.
ETA There is no place in Church teachings that I have ever come across where frequency of intercourse is ever particularly addressed in terms of how many times a week is allowed. A couple is being “moderate” (the admonition in the CCC) if they are trying to match up their intimate life with the practice of virtue. End of story.