Yeah I always thought the converters weren't worth it either, especially since I hear some of them can be fragile and freeze the game from the slightest tap.
I bought a Duo RX because of the trouble the orange Kisado(supposed to be the best) gives me with the Arcade Card Duo.
The only reason I have the Kisado, is because I bought it along with SFIICE as soon as it was released to play it on my Duo, TGCD and Express(got it in the mail the morning of a week long road trip) back in the day.
Back then, there was no real internet and import hardware cost a fortune.
Now there's no reason to buy a convertor other than as a collectible, especially with people like DLite providing quality mods for less.