While others fiddle with the mechanical aspects of a versatile bit of magic, this answer will strive to deal with the personal aspect of it.
A magic system requires not only the power and the rules surrounding it, but people to use it. Every person might be theoretically able to do everything that the magic can do, but it does not mean that they have the ability in practice.
To us that create it, the idea that you can create weapons behind somebody and launch them might be obvious, but it might not be an obvious or easy thing to the people of the world.
People Specialize
Just because it is theoretically plausible to do something, does not mean that everyone can do it. Polearm sodomy is a pretty specific skill and one that really only has the purpose of assassination or slapstick. Even then, there are likely better ways to do it with enough creativity. Also, not everyone has the mindset to summon polearms out of nowhere and throw them at people. It takes a certain kind of hero to do that.
That typed, you will have the assassin that fights by trying to launch weapons at blind spots, creating distractions, and otherwise using ambush tactics to win in a fight. Likewise, there will be someone that creates a giant golem to fight with while they stay preferably out of sight. On a more mundane front, you will have people that put White or Black magic to use in mundane pursuits such as farming or blacksmithing.
Most people will figure out that they are good at one or two things, then hone it into something that they can use on a regular basis. Some will choose to generalize, and they will be dangerous for the ability to do more, even if it isn't as masterful as what others that specialize in it can do.
For an idea, look at the secondary cast of Naruto -- chakra there can basically do anything. While there are a few set "basic skills", the characters have their own specialties and generally do not stray from it, though they may elaborate on them. Affinity and ability play into this, but rare is the character that is more of a dedicated generalist in the chakra arts there. Likewise, Hunter x Hunter is similar, with basic techniques that everyone can in theory use, and a personal skill that a character develops then elaborates on as time goes on. However, a character tends not to create a second all-new skill. They can however, apply the one they develop in various ways.
Affinities
Related to specialization above, people might have an inborn ability to a certain type of magic (white or black) as well as a certain way of using it. Related to the idea above that people will gravitate to a small subset of magical ability, this actually codifies that into a restriction of the system.
Depending on your desire, it might be possible to learn a magic that lies outside your affinities. Alternatively, it might be next to impossible to do so.
In summary: While the system can do almost anything, a person can't.
People are Uncreative
Avatar's bending is quite versatile if you think about it -- one can achieve a lot with control over one of the classical four elements. But if you watch the show, a good amount of the bending is similar to each other when seen in the show. Of course, there is only a small subset of the bending world shown, so what lies outside the told story could be different.
If your black and white magic is only passed down through tutelage from teacher to student or master to apprentice, then those students might not really think about using this magic in different ways. People will get used to the standard forms, and build a personal style based around it. Only a few might experiment and try to create something new and/or powerful as a personal skill.
This will go doubly so if the authorities that teach magic can arbitrarily decide what is taught and/or allowed to be known. Not that does not stop secret assassination techniques, but they won't be common because they won't be taught. Also, formalized teaching might start by teaching self-defense and shielding before moving on to attack skills which means that by the time these people grow up, a full-body defense might be a normal thing.
This is not necessarily a sleight against the people of your world -- it's just an application of the saying "If it ain't broke, don't fix it".
Personal Ability
Obviously not all people are created equally. Some will have more power, while others more control. Not everybody will be actually capable of doing everything, and most of this can be increased by training should somebody actually want to.
If willpower is needed to gather and shape magic, then there will be some people that can smash with large blocks of power while some could only create a single personal item to use/launch.
Capacity to wield magic can be another factor. Since it looks like this style of magic works by manipulating a physical substance, the amount that somebody can use is a personal skill. One that people can probably train to be better at.
Ability to output magic is a third potential parameter -- how much can a caster wield at a time. Somebody that can keep an item conjured for hours will have a different experience than someone that can create a giant golem but only for five minutes at a time.
