You are a digit
It was for me that modern quantization was named, and I'm important to theorists of certain persuasions.
Digits are used in counting, and ratios of fingers are studied by "digit theorists" (yes, it is a thing).
Puzzle creator answer: the whole idea of digital instead of analog is to define discrete steps or islands within the space of infinite variability of a signal or value, then round off the analog values to the nearest island (or “digit”). In a sound wave for example, rounding when done horizontally in time for the purposes of music creation is called quantization, and when done vertically it is what is normally thought of as digitization. Math is a highly theoretical discipline and I’m pretty sure they use digits in it. (Smile!)
Given a space, I can say I greatly enjoy it!
Insert a space, and As Tim Couwelier commented, you can "dig it"
My companions I'm never far from, and together we labor—though one may oppose me.
The thumb opposes the other fingers, and in some simians, one of the toes may oppose the other toes.
I bear the moon near one end, and a tree at the other.
Moon = cuticle(lunula)/nail; Tree = Palm
I bend in the middle, but what makes me move are ten of one thing coming from fore of another.
"Ten"dons come from the hands and feet to allow the digits to move and bend, as well as the "fore" arm.
Some would consider my weapon to be weak, but still, it can wound deeply. An insult delivered is never followed by retraction.
Pointing the finger in some societies is considered rude.
Puzzle creator answer: a human's finger nails don't compare to the claws of an animal, but can still scratch hard enough to draw blood. An "insult" delivered by fingernails (a scratched wound) is never followed by retraction of those nails--humans do not have retractable nails.
The wind blows across your face and I can make a blow on your face, too—but only with my friends.
You need your digits to create a fist (or slap).
Life is pointless without me.
You can't point without me (well, you can, but semantics...)
Alone or with others of my kind we can compose a feelingless, prickly weed seed.
~ Numb burrs ("numbers") [Thanks to Fillet]
I take planes into flight and feed hungry babes.
Difficult to do things without your digits.
I give ability to athletes as well as the deaf and the blind.
Athletes generally use their fingers/toes, and so do the blind and deaf (to read).
A wave of happiness takes over me from time to time.
Jazz hands/spirit fingers.
Puzzle creator answer: Simply waving at someone you're happy to see is all it takes for a "wave of happiness" to take over your digit(s)!
What am I?