I am trying to figure out why I would use this virtue.
Say I want to create an item which gives light. I would cast Sustain Spell of Fire at level 20 and expend 4 pawns of vis because it is a ritual. I then cast my light spell (less than level 20) and then bind a spirit to the item. The item will continue to shine light until I release the spirit which cancels the spells. If I want to then have the item create light again, I would need to cast Sustain Spell of Fire again, expending 4 pawns of vis and follow that with the light spell and bind the spirit.
Going the enchanted object route, I could just create an enchanted item that gives light, for likely the same cost of vis or maybe even a little less.
It seems both approaches require vis. The main difference, other than that whole spirit thing, is that creating an enchanted item takes a full season where binding a spirit is just a ritual spell.
Is that correct? Or am I misunderstanding something?
Nope, that's it. So basically you don't want to bind in effects you tend to cancel, unless you're swimming in vis.
The main use-case for Spirit Binding is penetration.
You're right that you can simply make an enchanted lamp for less vis, and while there's the odd case where Spirit Binding will let you make one faster, you still have to research the spells first.
However the moment your permanent-duration spell needs a penetration total, Spirit Binding becomes amazing.
For example, your spirit-focused magus comes across a might-50 storm spirit.
To bind this to your service with an item, you'd need to make a variant of 'Coerce the Spirits of Night' for storm spirits in the item then invest additional levels for penetration. The final item level is north of 50; that's an epic item.
If you later come across a might-60 storm spirit and want to bind it, you need to make a whole new item spending whole new seasons.
To bind this to your service with spirit binding, you need your variant spell and a weaker spirit you've also coerced to act as fuel. You then cast your binding spell as a spell, and get to use your own penetration total, including the benefits of arcane connection multipliers, to bind the big storm spirit. At no point did you cast a spell above level 25.
If you come across a second might-60 spirit, you can just use the exact same spells as last time. Which means that if you have a suitable might fuel-spirit to hand then you can nab the second one in under a day. Possibly even in an hour.
So in short: yes, you were correct. But it is possible you were missing something.
A-yup. That's what I was missing. That's pretty sweet.