Sure I’ll say it. But as an option!
I’d add more to this.
1. Combine these options into an area with 'Generate Spine' label, to explain these to new users
And shorter names, with more explanation displayed on hover:
— From Unpainted Path (checked)
— For Fully Nested objects
— For Intersection-based Nested objects
— For Open Paths
3. Add a button there (and / or an option in Object > Blend) — Reset Spine (guarded by a confirmation).
4. Add Release / Expand into the dialog finally (but this is beyond the spine question, requested separately: http://illustrator.uservoice.com/forums/333657/suggestions/31988758)

So apparently there are *four* scenarios wherein blending won't create a spline:

A) If there are at least three paths selected, and at least one of them is an open, unstroked, unfilled path, and at least two of them are not, then the open unpainted path is used as an initial spine and the other objects are moved onto it.

B) If the bounding box of the anchor points of each object encloses the bounding box of the anchor points of all the objects above it in the stacking order, then no spine path is created. (This is the ‘nested objects’ case.)

C) all centers are enclosed in the intersection that is built by of all objects

D) If all of the objects are open paths.

(see https://community.adobe.com/t5/illustrator-discussions/why-is-my-script-undoing-half-its-work-before-it-s-done/m-p/15443823#M448826 - thank you very much, Monika)

A is the only one of these that seems at all useful to me. That one's actually kind of neat! It's a shame it's documented absolutely nowhere that I can find. B, C, and D are aggressively useless to me; I wrote a script to deal with D and it makes my life easier but if a tight blend happens to trigger B or C as well as D then it's super annoying to me.

Document all of this. In the form of some switches in the blend tool's options? Somewhere else? Let me say I always want a spline. Let Egor say he never wants a spine.

I've attached a quick mockup of a revised Blend Options that shows all of these options, set to the default behavior. I can't quite parse what C means so the language for that one is ultra-vague, I'm gonna hope there's probably notes about the precise intent of that in the thirty-year-old source file that defines this behavior. :)

My script stopped working recently for some unknown reason. I can't figure out what the new criteria for making a spline or not are, some paths just do not want to blend with an automatically-created spline, and I am so tired of working around this behavior.

Give us some controls for this. It's *so* much easier when I can just draw a few paths, blend them, and immediately push the spline around, I *never* want to not have one created and I *never* want to have to create one myself, let me tell Illustrator to always generate one. And let Egor say he never wants one, too! Stop trying to guess based on some unspoken criteria. Give us a nice little set of controls to express our preferences for all the combinations of "always/never create a spline for blends involving open/closed paths".

Actually, I think I need an option to never create a spline also.

I got so annoyed by this behavior that I wrote a script to work around it. Now I can just hit command-shift-alt-b and get a blend with a spline, no matter what the paths are - if the first path's open, the script closes it, blends, and opens it again. I can finally blend paths without cursing.

https://egypt.urnash.com/blog/2023/12/04/fixing-an-illustrator-bug-blend-open-paths-with-a-spline/

I really hate this. Whose workflow of only wanting a spline when they blend between closed paths was this designed around? I work almost exclusively with open paths and it is just *super* annoying to have to either swear, add a blend spline after the fact, and move the start and end points, or swear, undo, close one of the paths, blend, and open the paths again. I want to be able to draw two paths, select them, make a blend, and immediately get to pushing the spline around without all that hassle.

If someone has a good case for only wanting splines generated on closed-path blends, fine: make a switch somewhere to let me choose between "make splines for all blends/closed-path blends only/open-path blends only".

I just had this problem, I have used Illustrator for 17 years and always I could use blend tool without problems. The spine is essential to create effective shapes, without a spine the tool is useless :(