Based on my knowledge as a biologist I would like to add some comments. If you would look at what the leachate could contain it would be the following: water (duh), salts, stable organic compounds and microorganisms. Let’s break that down:

Water: Most of it, adds close to nothing benefitial in itself.

Salts: The effect of salts could have a big effect on the use of the leachate as it infuences everything a lot. Too much salt could harm the plants and microorganisms in the leachate. One question is thus how salt leachate actually is. Various sources always say to dilute the leachate before use for this reason. If the amount of salts is not too high, it is actually beneficial to plants as it contains trace elements and provides a (indirect) nitrogen source for take up.

Stable organis compounds: Like said before above, older bins have most likely more of these compounds than young bins. These are the same compounds that could be found in forrest soil and compost. Simple organic compounds, like sugars, will most likely not be present in leachate, as microorganisms quickly absorb them. These compounds have most likely little infuence on plants, but could improve the microbial soil community (which indirect would be beneficial for plants).

Microorganisms: Expect them to be present in the leachate, but I doubt they will do much in the leachate. The reason for this is that carbon is hard for them to get; no sugars and the stable organic compounds are much harder to degrade for carbon and energy. A high salt concentration also inhibits growth of most soil microorganisms. If the collection container is closed (and an anearobic environment is created), growth of microorganisms will be inhibited as well. Anearobic microorganisms will be able to grow but will run into growth problems as well for the reasons mentioned above.

So, in perspective. I think the longer the leachate is sitting in a closed bottle, the less benefitial it will be. I doubt it will be really damaging to plants, unless the salt (mostly sodium and potassium) concentration runs through the roof. The best proof of the beneficial properties of vermicompost to plants are the trench examples on this site.