Since this was twice as long as the Minecraft Feedback site allows, here's all of my reasoning on why to include a 10th signal strength for having multiple items in every slot.

I like most current features of the Crafter. However, the block could be easily made more accessible to non-technical players without a new addition such as a "crafting blueprint" or other recipe-locking mechanism: tweaking the comparator output. I feel that the comparator signals 1-9 work very well, but I think a 10th signal is necessary to indicate when there are 2 or more items in each slot. With this change, it will only take one circuit for many complex recipes to work easily. This could ensure that the crafter never loses its recipe when used properly, and it still functions without the limitations of a recipe lock, template, or blueprint - those limitations being a lack of freedom to technical players who intend to use one crafter for multiple recipes. It also feels much more Minecrafty to have this controlled by a comparator, rather than a UI feature.

This also makes the crafter useful on a wide scale. If the crafter is implemented without a system like this, I personally will make an advanced crafting recipe maker in my long-term world once, as a novelty. Most non-technical players will probably never use it beyond resource compaction, at most. It would be too much effort to use it for anything else, especially if every recipe requires a different circuit. However, if the 10th signal strength is introduced, many players would be able to use the Crafter in many ways without fear of wasted time or resources from broken recipes. One example I point to is firework rocket farms: they collect gunpowder and sugar cane, and the sugar cane can become paper using the current system. However, they can only be reliably crafted together using a fail-proof system: a mass of redstone for this single farm, or one all-purpose circuit. This would lead to the farming community accepting the crafter as a significant part of the game.

One small argument against this design is the presence of unstackable items in some recipes, such as the milk buckets used in cakes. However, I believe this should be dismissed since unstackable items naturally lend themself to being more technical in item transport systems. Some examples are item sorters, automatic storage, and even basic hopper lines at times. Technical players get over these advanced problems with technical solutions, and they always will. Players who are eager to use unstackables still can, but those who don't shouldn't have to go through loads of effort just to have fun with a Crafter.