Toner is much more complex than just powder.
Toner is not just toner. There are many different toner technologies. Some toner is chemically processed, and some is not. Toner has to be compatible with the components its works with. It must have the proper flow agents, so the toner properly transports. Some toner is horrible quality. Some toner will have multiple sized microns of toner particles. Some are too coarse and some are to fine. This will impact the electrostatic charging. Some particles will charge and some will not. Sometimes toner is not compatible with the magnetic roller and flow agents will separate cause a film on the mag sleeve. This will create a resistant barrier causing the toner not to charge properly. This will normally happen within 1 to 2 thousand pages. Toner has to have a proper melting point. It a incorrect toner is used it cause the toner not to fuse properly to the paper. This toner will build up on the fuser unit and damage the fuser. Some fusers only work well with chemically processed toner. I am only listing a very small percentage of potential problems.
Toner is not the only component in a laser cartridge that should be replaced.
There are many other components in laser cartridges that wear due to friction. OPC drum typically only last one cycle. The charge generation layer will wear differently depending on usage. It printing is done continuously and toner is layed down on the page evenly throughout the page then your drum will survive much longer. Turning the printer on, off, then on alot or printing in the same page area will destroy a drum much faster. Drums are cleaned by a wiper blade which does most of the damage. Wiper blades typically have a power type lubricant on them to reduce friction. Eventually the wiper blade may also wear but it will last longer than the drum. A primary charge roller charges and conditions the drum. pcr normally survive more than one cycle but they may have a mid-cycle failure later on. Most cartridge remanufacturers will replace them with new ones or have them recoated with a protective layer. Magnetic rollers have a fine abrasive layer to produce friction and also have a conductive coating. This charge toner particles also with the help of a doctor blade that rest against the mag roller to also help produce friction. Eventually the coating will wear. Without the conductive coating the roller will oxidize causing very light print. Also the doctor blade wears quite a bit.
Shelf life of a toner cartridge varies but it is safe to say 1 to 2 years depending on the cartridge and the environment the cartridge is stored in.
If toner comes in contact with any abnormal humidity or temperature it will degrade over a short time. Toner particles will stick together and produce bad prints. The primary charge rollers (PCRs) rest against the drum the whole time in storage. The PCR has chemical called plasticizers within it that keeps it plyable. Eventually the plasticizers will evaporate causing the PCR to harden and crack. Also the plasticizers will contaiminate the drum with one line across the drum. Also if the PCR rest against the drum to long it will leave a flat spot in the pcr causing improper charging to the drum.
This is all I can stand to write, but this is only a fraction of a percent of potential issues and requirements to reusing laser cartridges.
So should you use refill kits (known as drill and filling) which on replaces toner with who knows what toner and what toner manufacturer it came from. If you really just don't care about quality or if you are not worried about damage to your fuser unit, then go for it.