I agree Legere makes great synthetics. I use the Signatures Series for my SA& T saxes, as well as bass clarinet. For my Bb clarinet I use European Cut. However, I am not a fan of their bari sax reeds. That's why I switched to Harry's Fiberreed. It works like a charm on my Couf. Gives me the sound I want when combined with the Theo Wanne Durga 3 MP I use, and is incredibly flexible. But... Finding the right strength is the issue.
I used to use the original Fibracell reeds on bass. Sadly they all finally wore out, and the "new" ones--anything produced over the past 12 or so years--are all sh*t in my estimation. They sound horrible, and the delaminate in no time. They also no longer have the staying-power they used to.
That's how I ended up switching to the Hartmann reeds on bass. I was desperate to try a reed that actually wasn't worn out for my new Couf bass, so I grabbed one of my bari reeds. Worked fantastically on the JK-made horn.
I am one of these players who likes their reeds to be consistent from reed to reed, and to play effortlessly from the get. That's why I use synthetics. Players I work with are always surprised to find out I don't use cane reeds. It doesn't effect the sound out front, but may effect what we as players hear--at first.
They certainly respond differently, and I get what you say about the sponginess. The carbon onyx is not nearly as stiff as a cane reed is, but that is what I use to create a lot of the tonal expressions when I play. In other words, I use (insert synthetic reed of choice name here) to customize the sound and effects I get from my various horns.
By far and away most players I know are cane purists
sax players for hire. I get that. I think I spent too many years playing in rock and electric blues bands opposite the lead guitar player where I blended my sound to theirs to the point where you couldn't tell where the guitar solo ended, and where the sax solo started. These 20+ years of working like that still effect how I use the tools in my toolbox today.
Although my sound is vastly different in a 19-piece swing band than it was in a 5 piece rock band, I still dip into what is comfortable for me, and what gets me the results I need.