In the past, stories were largely random and aimless, spur-of-the-moment decisions and whatnot were what drove the 'plot'. Write-ins in these days were basically the readers actively writing the story with the writer, and they were the norm, about as frequent if not moreso than 'default' options. Heck, many stories have been written solely through write-ins alone, successfully.
Nowadays, though, stories have changed to the point where most of them are plot-driven from the start - instead of 'guy wakes up in Gensokyo, wat do?' we have a story, established characters and personalities, pre-planned plot events and even endings. As such, the stories themselves are quite a bit less flexible, in a sense. Not every vote, nor even every story can accomodate write-ins anymore. Often, this is mentioned explicitly at the start of the story itself, but in some cases it's never brought up.
Now, what should the regular assumption here be? Are write-ins opt-in or opt-out; as in, are they assumed to be allowed by default, or should they only be given if the author prompts it?
There are certainly cases to be made for both viewpoints, as was done to a degree in the aforementioned thread. Discuss.
The way I see it, unless they are explicitly prohibited, there is nothing wrong with at least trying for a write-in, so long as it's understood that there's no guarantee that it will work out, and that the writer is not obligated to use it if it can't work in the story.
Though more stories these days are more rigid than they used to be, that doesn't mean they can't benefit from an impromptu write-in. Though a writer might have a vision, or a specific idea of what they want to do, they may not have considered every way in which they can realize that idea. In those cases, a good write-in might give them an idea they may not have thought of on their own, and provide them with a new direction to take that actually works better for them than their original one.
On the other hand, if the write-in isn't workable, the writer can always simply disregard it. If, at that point, the writer does not make a stand and say "no more write-ins" then just take it for what it is. You had an idea, put it forward, and it simply didn't work out. That's just how it goes with ideas and suggestions.
In short, unless outright prohibited, there's really no reason not to offer a write-in if you think you have a good one. The worst that could happen is it isn't accepted, and if that's what happens, you just move on and hope for better luck the next time around.
Personally, I think the worst thing about write-ins isn't so much that they might not work with a story, but the attitude the people voting for it have that it will work because it's a write-in, with little or no regard for the context in which the write-in is being made.
Say you have two characters in a story who don't like each other. Perhaps even hate each other, so much so that it's impossible to have them together without a fight breaking out. Yet, through the events of the story, the protagonist has wound up in a position where they deal with both of those characters on a regular basis, but never at the same time.
Suppose, then, that something happens that winds up forcing the protagonist to have to deal with the two of them at the same time. Everything that has happened up to that point to show you what these to characters are like indicate that the two of them meeting will not end well. The narrative even might go so far as to explicitly state it won't end well, and the options provided more or less boil down to either helping one character hide or slip away, or distracting the other character and getting her to go away.
If you decide to disregard all those little hints of "don't get these two together yet, it won't end well" and construct a write-in that forces them together, you can't really blame the writer for not forcing it to work out how you intended it to. Blame them for not simply vetoing it, maybe, but not for making your vote work out in a way that wasn't how you wanted it to work out. A write-in is simply an alternate way of dealing with a given scenario, not an "Instant Win" button.
A similar problem arises in write-ins, particularly the long ones, that propose a series of actions that are completely dependent on a certain series of reactions happening for it to work, like a long string of if-then statements put in narrative form. It assumes too much to think that just one thing you write-in will work out in a certain, specific way, let alone an entire series of them.
Just because a write-in is made with a certain intended result in mind does not mean that result can or will happen.
Hahahahahahaha. A write in made in an attempt to avoid shitstorms the normal choices would have caused instead results in making more shitstorms than either of those choices. Thanks I needed the laugh. Then again, shitstorms/debate was part of the plan with such polarized choices.
But on the serious side, a writer should be aware of their right to turn down a write in if it's crap. As far as what if votes? some think that further detail are needed, but it's also a sign that they don't trust the writer to handle things. Ideally, a story can function without them, but they might be able to get better results than planned.
On the flip side making things more write in heavy in an obvious attempt to get attention doesn't work well as it puts anon on the spot and increases the risk of bad write-ins. It also reeks of the author having no plan/idea.
Write ins are a source of excellent new ideas and paths. They should be kept short and related to an existing choice, to give the author a chance to cop-out if it is too outlandish or goes against his plot.