Just looking at the links you've provided, I think the answer is evident in at least the first and last questions as they've been closed. The last one in particular is clearly too broad and therefore invalid.
The first one is a little more subjective. One could argue that it's a Physics SE question dressed up with a knight to make it fit WB, but technically I think it was a valid question. It was asking for science based answers to a specific world based question that didn't ask us to 'write the story for him/her'.
I'm not saying that it wasn't ridiculous, and a part of me thinks it's more the domain of Randall Monroe on XKCD but I'd also point out that prior to it being closed, it garnered 3 answers, so clearly there were some who were prepared to take it as a legitimate question. The last one, about biology and evolution, didn't attract a single one. That could be because it was closed quickly, but given that the middle one is still open and has 5 answers against it, I'm of the view that relative legitimacy of a question can at least in part be seen as correlating to the number of answers provided. (This is a small sample set admittedly, and it would be interesting to see some figures provided on this across a larger sample of questions.)
I'd also point out that 'too broad', 'primarily opinion based' and to a lesser degree 'off-topic' can be taken as subjective assessments in many cases. I've seen questions closed as too broad that I thought were quite specific. I've even answered some of them before they were closed.
To that end, even this question on meta could be argued to be 'primarily opinion based'. :) I think like Kingledion says in comments, there are some questions he simply ignores. The fact that questions don't get ~100 answers on a regular basis tells me that we all select what we're prepared to answer to some degree, and I also think that in the spirit of this question, it's important to note that not everyone who comes to this site to ask a question has a detailed background in biology, physics, engineering, economics, anthropology, or even [insert your specialisation here]. As such, without trying to take ourselves too seriously, there is a compelling case that a part of the responsibility we have here (as a tradeoff for the fun of it) is to teach.
As such, while some questions are just begging to be closed, I think it's entirely legitimate to answer others, if for no other reason than to guide the poster of the question. Exactly where the discriminator lies between these conditions is a personal choice. Ultimately this question is testing the boundary conditions of something that we all already do;
If you feel comfortable answering a question, do so.
If you don't, well, don't.