The word “witnessing” as used there has a very specific meaning - it’s limited to “Come to the Lord,” type posts. We want those initially placed in GD, because in our experience, that’s where they’d all end up anyway given the contentious nature of religion. It’s not necesarily that MPSIMS is unsuited to witnessing OPs, but rather that the replies which are typical of a witnessing thread are better suited to GD. Also, over the years, GD has evolved some rules—formal ones enforced by the staff, and some informal ones to which a poster is expected to conform by the GD denizens—that are particularly suited to threads about religion, so it’s better suited for those topics. Note please that wherever the word “religion” has been employed above, it could likely be replaced by “politics” with little difference.
MPSIMS is more for light banter; the phrase “great thoughts” is kinda misleading. While the over all tone of is general chatter, there are many topics which are anything but. I’m thinking of things like remembrance & memorial threads, the threads where the community comes together to help members (and sometimes others, or specific causes) in need, and things similar to that. Not things that are necessarily “great thoughts,” but things that are certainly above the level of light banter and humor. Maybe we need to consider changing the MPSIMS forum description.
IMHO was initially set-up as a place to put things that we saw falling between Great Debates and MPSIMS. It was intended to take some of the load off both of those forums. Hence, it’s the designated place for polls. It hasn’t worked exactly as we anticipated, but it does provide a place for some unique topics. As is obvious, Great Debates would be the place for something like arguing the merits of Mahayana vs. Theravada Buddhism. IMHO would be appropriate for asking the members preference between Miracle Whip or Hellman’s.
As for your specific example of “I think Senator Smith Should be Re-elected,” my best guess is that it might be okay in IMHO, but I wouldn’t be at all surprised if it was moved in rather short order to GD. That would likely depend on, as I noted in my first paragraph, the responses engendered. In any case, however, one can never go wrong by writing to the mods of the forum(s) for which you think any proposed thread might be appropriate. Also, the mods of the three forums with which you have expressed concern, know that there are topics that may cross over the boundaries. We don’t get too testy if we have to move a thread. It’s the forums which have clear boundaries—GQ and the comments on the columns, where we get more exasperated at finding misplaced topics.
I hope this helps and if you have more questions, please ask. We’ll do what we can to answer 'em.