There are a number of CD titles that need the 6-button controllers. Of all Hucard releases, only Street Fighter II Champion Edition and it's sexy knockoff "Strip Fighter" actually use the 6-button control scheme. I own Street Fighter and it's a PITA to try to play with a normal controller. No pause function, Run is your third punch/kick, and Select swaps between kicking and punching. That said, my Cthulhu arcade stick works nicely with it as it supports 2- and 6-button control schemes, even though I tend to randomly mash buttons and generally suck at fighters.
However if you're going to get a 3rd party controller, I would recommend an Avenue 3 controller over the Avenue 6 as it's a lot more useful in many games in that you can map Start or Select to the third button making certain games more accessible. Activating turbo-hold function on button III can also serve as a kind of ghetto slo-mo in certain games. It really depends on what you want to play, but the 6-button controllers IMO are getting kind of spendy now.