I buy from Open Market occasionally, usually hullmods early in the game or cheap weapons I need now. If I am desperate for supplies and fuel, and Black Market does not have enough, then yes, I pay them the blood money at Open Market.
Wait, there is another use for Open Market - selling pristine nanoforge at New Maxios (after I steal the corrupted nanoforge) so I can raid and steal it back, and repeat, for profit! It is a royal pain to steal back the nanoforge, and after trying it once or twice, I decided it was more trouble than it was worth.
Overall, I rarely use Open Market. It can be removed from the game, and little will change.
I avoid commissions because I do not want the "Commissions" tag permanently branding my Intel like a scarlet letter, which means no military markets for me.
Think of it this way: Why does the buffalo mk2, cerberus and hound exist and the pirates don't have better ships? Because you need contrast.
Pirate ships are NPCs and UI is irrelevant for them. Open Market is a blatant (newbie) trap at every colony, and it is the default market to shop at. (It is the main reason why I refuse to put Commerce on my colonies, because I do not want default markets changed to Open Market at my colonies.)
Military markets are also almost useless if you aren't commissioned. The size limit on what is sold doesn't makes sense. Want to buy a Buffalo? Commission only!
They also barely have more than Open Market in the more recent releases. They used to have lots of good stuff, but not anymore.