Just thinking about reusing the kaweco cartridge that came with my skyline sport, how many times can I reuse if filled with a syringe? I mean some of the cartridges tend to crack at the mouth. Almost all of my LAMY cartridges and several other brands have cracked after reusing for a couple of times.
Some people here on FPN refill cartridges all the time with no problems. I prefer not to refill. I only use cartridges in my two Kaweco Dia2s & my Lilliput. These pens are not used very often.
I have found that after a few refills, a cartridge no longer fits as securely at its mouth. No matter if the cart was made by a reputable manufacturer, the plastic at the opening gets deformed.
It's like removing & re-attaching your converter every single time you would fill a cartridge/converter pen. Your converter would eventually leak at its mouth if you regularly pulled it out & plugged it in.