I like the month by month membership because, as you said, there are certain times where you might not be using Subtext as much, especially if you plan your project in advance.
This year, I put my membership on hold for a few months while I finished one story (big sci-fi novel). That one I had planned and understood enough that I didn't really need Subtext to finish it. (I did make use of the treatment I had exported from Subtext, of course.)
Then, I started a new project but didn't need Subtext immediately as I was still coming up with ideas for it. Once I started to sense a premise/storyform in it, I renewed my monthly membership. At that point Subtext was invaluable at convincing me I had the right storyform (because the PSR gists were so accurate).
For this current project, I have done less planning, almost at the verge of "pantsing" it. Because of that, I have found that I'm going into Subtext a few times a week to update illustrations of beats that I have either recently written or am about to come up to. Subtext is helping me to sort of see and understand the story on the horizon, to see which of my ideas fit the story better. And to give me a nice pat on the back when I realize how well something I've already written fits an existing beat (though I didn't realize it at the time).
Note that because of how I use Dramatica and develop story ideas, I think I would always figure out the storyform (or a small number of candidate storyforms) in Dramatica first, before importing them into Subtext. I can't really imagine trying to base a story off of an existing film's storyform, unless I was literally going in with no ideas at all. I feel like by the time I get them, my ideas already have a storyform (or small number of potential storyforms) baked into them.