In my school the week or so of cover would likely be covered by one of the school's own cover supervisors, who have full time roles at the school and know pupils and teachers well, unless for some reason that week an unusually high number of staff were unexpectedly absent. I realise that this won't be the case in all schools, but for the short period we're talking about, I still don't think it would be as chaotic as you suggest.
Of course, I'm not saying that classes having a small amount of cover is the ideal, but it's far from disastrous. I don't miss my children's school assemblies, sports days etc if they fall on my working days - I get cover, set work and go to them without any guilt, as do other colleagues, because my current school/SLT are openly very supportive of this. (I realise I'm lucky in that respect and not all schools are the same, as I've previously worked in schools where this would not be permitted).
You obviously think some reasons for using the leave are acceptable, and some, such as family holidays, aren't. But the fact is, teachers, like all parents, are entitled to use the leave for holidays or however else they wish.
Your comments come across as a bit judgemental of/guilt-tripping other teacher parents about using their entitlement. The 'but it's all for the kids' aspect of our job already expects us to go 'above and beyond' and has us giving so much of ourselves to the role. Teachers using unpaid parental leave is clearly not something you agree with and that's fine, you don't personally have to take it.
Anyway, I don't want to derail the original thread too much so I'll leave it there.