About the only reason to buy 3rd party inks is if you are interested in saving a "few bucks", or to print B&W with an older Epson printer with pure black carbon inks of various dilutions.
I think part of the reason there are so many Epson ink knock offs, is because Epson printers require much more vigorous cleaning cycles to keep their nozzles clean, and they (Epsons) are notorious for leaving a considerable amount of ink in an "empty" cartridge. Epson could easily have changed their pricing policies to fairly compete with those objections, but obviously Epson hasn't.
If you are in business to make prints, then your prime objectives are to have a printer that is dependable and trouble free. Having owned many Epsons dating back to the original 9000, I find it interesting that not much has changed regarding using Epson printers--most people still run nozzle checks before making serious prints. Having owned an HPZ3100 for 3 years now, I find it comforting that I don't run nozzle checks! The printer is just always ready to print without any fuss or bother. The "reject" print due to printer problems is the rare exception with the HP.
Although 3rd party inks are available for the HP, and probably the Canon printers, I haven't read any posts of people using those products. Why is it that Epson users seem to be concerned about ink costs and using 3rd party inks?
In another life, I was in the 3rd party ink business for the Epson 9000 printers. Remember, they only could use dye inks, which faded fairly rapidly when not used with swellable polymer ink receptor media. I had an ink chemist modify Ilford dye inks for "thermal printers", which Wilhelm found had a lifespan of up to 70 years on specific media using 300 dpi Encad printers. The only reason I offered this for sale, was because Epson had no inkling of the possibilities of the lifespans required for fine art printing when they started marketing the 9000 printer back in 1998. The first pigmented 3rd party inks from Media street, compared very poorly in gamut and fade resistance, but people believed that pigments were more fade resistant than dyes.
Epson caught on pretty quickly about the need to use pigmented inks in these "professional" printers. But their first effort, the 9500, results looked more like "prefaded" dye inks, lacking color gamut. They finally got their act together with a good combination in the 96/7600 series.
But back to the use of 3rd party inks in today's pro printers, I simply ask "why"?