From what I have learned about foliar applications, is they are very good at a few things, in the correct circumstances. I am not sold on them, yet, for broad-acre food plots/pastures but I am hoping to learn more and try some of my own experiments.
1. They are often formulated with a base of Humic acid - which is nice, as it is not going to have a salt/oxidizing effect on your soil - as standard fert does.
2. They are very good at feeding the plant, directly throughout various times of the year. Also delivering the exact missing unit, in an already available form. Let's take iron, for example, it is often in our soil but already in an insoluble form of ferric oxide due to years of redox reactions occurring by man, naturally high PHs, etc. If we can identify FE as the lowest hole in the bucket (Liebig's law of the minimum) - we can have a tremendous crop response, as we are giving the plant something that it was struggling to gather in chelated form - so it is readily absorbed by the plant.
3. Dr. Christine Jones referenced in a webinar that much of the compost teas, extracts, etc. are mostly just microbe food -I still see value in this, if your Carbon to Nitrogen isn't balanced in the beginning years and you need to enhance microbe activity to reduce the carbon load on the soil. Also, as we know - microbe activity increasing, with plant growth increasing and exudating - more good things happen.
All in all - I think there is some for sure value in foliar sprays but I also believe it to be highly variable on the situation.
For example, If you have a year where the deer just absolutely decimate a field, I think adding a biological primer to boost organic N, feed microbes, etc. could be beneficial - as you probably had some die-off due to the plants stopping exudation (Green Cover Seed 7th edition talks about browsing and plant exudate production when over browsed). I also think that if you have a good level of microbial activity and you just want to add a different level of bacterial activity by spraying a fish hydrolysate or something like that - you can have some fun with it - as long as your goals and expectations are aligned.
Mostly, these are highly valuable for farmers (grain and/or produce) who are taking tissue sap analysis and can pinpoint a very specific mineral that is needed to be added and do so in a readily available form.
A few last items to consider and references:
1. John Kempf - he has several videos on this
2. Dr. Christine Jones - the Nitrogen Solution
3. Mulder's Chart
4. Plant availability by PH
5. Neil Kinsey - Hands-on Agronomy
With these things in mind, we can just try to understand the balance that we can achieve without inputs and how/what will occur when inputs are used. There are both synergistic and antagonistic relationships expressed, as it pertains to nutrient availability, with darn near every decision we make with our soil. So always keep in mind the law of the minimum, but never forget the law of the maximum. A quick example of the law of maximum, too much N that is not absorbed will actually take Ca away from our soils - this can impact PH and plant health, and the representation of the %s we want in the soil colloid.
I hope this is somewhat useful!
AT