Refilling Fountain Pen Cartridges

Do y'all save your spent fountain pen cartridges and refill them with bottled ink and a syringe?

I saw a video of Brian Goulet doing it, and it looks very easy. But I'm not clear on the utility if you have a converter for that pen. Faster loading of new ink? The ability to carry different inks with you in small convenient pre-loads (sealed somehow I assume)? Does it combine the ink-choice of bottles with the convenience of cartridges? Is this a common practice? Does the cartridge reliably seal multiple times?

I do save used cartridges..
However I use converters on any pen that I can, as they are usually more robust than disposable cartridges - but I end up refilling my converters using a syringe rather than using the twisty mechanisms (so basically I refill converters and cartridges the same way).
There are a couple fountain pens that I have that will not fit any of my converters - in those cases I have to refill a cartridge - but just the one I intend to load into the pen. I am not sure if there is a way to load additional cartridges beforehand and re-seal them for later use...

Like Luxem said, it is hard to get converters that fit some pens. There are even a few pens out there that take proprietary cartridges that are no longer made; so in both those instances, refilling carts is about the only option.

Obviosly, a bottle of ink is much cheaper than cartridge ink, and some companies (Sailor and Pilot come to mind) have converters that have miniscule capacity, but their carts hold substantially more ink. Refilling the cartridges from those pens allow the best of both worlds (from a cost standpoint)

I do occasionally refill cartridges for specific reasons. In the case of my Kaweco Liliput, there is no good converter solution, and with its metal body it isn't a good candidate for eyedropper filling, but I don't use the pen that much anyway. There are a couple of other pens for which I have no good converter. With some of the Pilot pens, as mentioned above, a cartridge may actually hold more ink than the smaller converters. Also, a refilled Pilot cartridge can easily be resealed with the original plastic disk, although on repeated use, the disk may not turn sideways the way it should when it's reinserted; I've had to pull them out again and turn the disk sideways with a toothpick so it isn't obstructing ink flow. Resealing an international cartridge so that it's safe to carry in luggage may be more problematic; I've never tried it. Nor have I tried it with proprietary Platinum, Sheaffer, or Parker cartridges. I've just refilled them and used them immediately.

When inserting a refilled cartridge in a "dry" pen, if I have a bottle of the same ink handy, I also prime the nib and feed by dipping them in it. Alternate methods to get the ink flow started, such as squeezing the cartridge or shaking the pen are less effective, in my experience, and may result in ink where you don't want it. On the other hand, leaving it with the nib angled down and testing at intervals while being patient will work.

In general, though, I prefer to stick with pens that have good converters or a built in filling system. It's possible that with some cartridges, repeatedly removing and reinserting them may eventually make them too loose a fit, and you'll have to start refilling another one. I haven't seen this happen myself. But I have seen both international converters and cartridges that fit on one pen be too loose when I tried to put them in another that nominally takes the same type. My guess is that this is because the nipple that they fit over is not the same size on every pen, and they get stretched a little. If you have empty cartridges lying around that you want to try this with, I'd suggest testing the fit with the pen you plan to use before refilling them.

I refill Pilot and Parker cartridges because they hold more ink than the converters I have for the pens.

I refill cartridges for my Pilot VP, since the con-50's capacity is nothing short of ridiculous. It's easy, I just use a blunt syringe and it takes a few seconds.

I tend to refill carts simply because it's easier and less messy than dipping the pen into the bottle. Only issue is that you have to let the ink saturate the feed down from the cart which can take a little time vs. being ready to write immediately when using a converter.

I refill used cartriges in pens, which converters don´t hold a lot of ink, like my Kaweco Sports, Lamy, Pilot or Platinum. I also use some pens for work, where I need permanent ink. I made the experience, that some converters die on you, if you use iron-gall ink in them. If I would use an con-70 converter with iron-gall, it stops working after the second or thirs refill. Even after the first refill you can´t see through the most converters anymore. I started to refill cartriges. If it gets to ugly, I can throw it away.
For refilling I use a small plastic bottle with an iron piece on top. It fits perfectly in every cartrige an it holds about 5ml of ink.

Thank you all for the many tips and useful information.