In my opinion, closing bad list-of or poll style subjective questions is a time tested method for improving overall question quality on stack exchange sites. This is opposite to what I thought when I first started hanging out here though. It's grown on me over time.
We do allow polls by the way, witness exhibits a & b, it's just that it is hard to get right (and I'm not sure that those two examples acheive that). There are structural problems with poll questions that Stack Exchange just isn't designed to address. They have a best before date, both in terms of what is best last year may not be best this year, and also in terms of user participation. The real problem I have with them best-before-wise is that they largely reflect only the opinions of those active in the few days/weeks of posting. It is very rare that a contribution made after the freshness period is over gets a fair viewing and voting in accord with it's merit. They encourage dozens to hundred of one-line answers that just aren't worth very much.
The Good Subjective, Bad Subjective blog post by Robert Cairtano is a distillation of community wisdom on the difficulties and constructive solutions to list and polls within the Stack Exchange network. It's long post, worth reading, but to get started here is a summary of the guidelines for Great Subjective Questions:
...inspire answers that explain “why” and “how”. The best subjective questions invite explanation.
...tend to have long, not short, answers. The best subjective questions inspire your peers to share their actual experiences, not just post a mindless one-liner or cartoon in hopes of being rewarded with upvotes for being merely “first.”
...have a constructive, fair, and impartial tone. The best subjective questions avoid the all too seductive route of ranting and flamebait. They set the right tone of constructive learning and collaboration from the very outset
...invite sharing experiences over opinions. Certainly experiences inform opinions, but the best subjective questions unabashedly and unashamedly prioritize sharing actual experiences over random opinions.
...insist that opinion be backed up with facts and references. Opinion isn’t all bad, so long as it’s backed up with something other than “because I’m an expert”, or “because I said so”, or “just because”. Use your specific experiences to back up your opinions ... or point to some research
...are more than just mindless social fun. Could an average member of our community reasonably be expected to learn something that makes them better at their job from this question?
