I'm the one who reopened it and I make no apologies for it. To make things clear I didn't discuss it with fellow mods or anyone else. None were on at the time or maybe I would have, but I wasn't going to wait either.
The discussions have already been had. Over and over.
Related Meta Discussions on Opinion Based Questions
So now the answer to your question, when should a mod use their power. When they see fit as PieBie said is the answer. But where PieBie says a discussion should be had and comments left I'll differ and say - a discussion was already had. Furthermore, in the exact example that you mention not one of the 5 people that voted to close it left any comment to the person that asked with a concern about it. Likewise on the question 3 weeks ago (What paper should a design print shop have on hand? whcih led to Why is this question about Paper Selections 'too broad'?) not one person that voted to close came into meta to discuss it.
But what feedback have we received as mods, during the election a number of members encouraged the mods to be more assertive with our moderation abilities.
Well, I saw no reason for the question to be closed. I saw no effort on the part of those closing it to explain why they voted that way. In fact we've been digging into close votes and some people don't even seem to read the questions just mass Open or mass Close (which we're preparing to address on an individual level).
We've already had the conversation about what is on-topic and what isn't. Part of my role as a moderator is to oversee those reasons. Where you see it as overstepping I see it as protecting the years that we've put into this site. Look through the meta - over and over members have wanted more Why questions, more good subjective questions. There are questions I don't agree with - a lot of the questions about automation I don't really think belong here. But the community does so I don't vote to close them, I don't hammer them, I don't even downvote them usually. I let the users that want them have them. If you can't bring yourself to do that regarding questions you don't entirely agree with than I don't think this community is the right place for you.
In summary you're asking why I reopened without discussion. Well there's been discussion I linked to a few of them above. I made one of them featured when I reopened it. The discussions been had. If members chose to ignore those discussions than that's on them. I'm not going to continue to ask the same questions to have fellow mods chime in then none of the non-mods comment and little change. The time for that has long past. It's been made clear that the community would like us moderators to do more because we don't have enough voters. So I stepped up and did. I appreciate that the other mods are stepping in and trying to justify this but at the same time, this isn't on them. It's on me. I did it and I'll tell you what - I fully intend to do it more regularly. I'm tired of fruitless discussions and watching good questions get closed.