There are so many GREAT players doing their thing with Spiros.
To me, it's because the string lets these players be heard over an ensemble. They give an unbeatable shine to the sound and let the bassist express so many different things without getting buried by the other players.
If that's the sound you want and you can get away with that, then I say go for. Most people though, including bassists, don't seem to care that much for it. They say they want that shine too, but what they really want is to have a sound that lets them hear themselves clearly yet brings that more muddy, deep, less distinct sound to the audience. Hence, the overwhelming advice about letting spiros age and get duller and so on. I can appreciate the desire to have it both ways, but I don't think it's realistic. I think it's more realistic to trust that what you hear is what others hear, and to strive for a sound that pleases you and supports your confidence.
I hear a great confidence in Ray Brown's playing. To me, Ray wants to be heard and wants to be part of every conversation, even when somebody like Oscar Peterson is talking! I remember the first time I heard an Oscar Peterson recording and my first thought was, "Who is that BADASS bass player!?!"
That wasn't my first thought when I heard Paul Chambers on a Miles recording for the first time. In fact, I didn't even wonder who it was until I heard the band make room for a PC solo...DAMN...and that's a GREAT sound too!
I've only ever heard 1 orchestra bassist play a bass with spiros, and I could easily tell when she was playing and what she was playing all the time. It was fine for me and she sounded great, but that's not the classical bass sound I'm accustomed to hearing. I've tried brighter strings myself (not spiros, though) in a classical setting and the conductor didn't want it at all. He didn't care that I wanted to rock with the bassoon player at all times. He wanted me to blend in and stay behind in support of as many players as possible, because that's how he envisioned the ensemble sound. So I gave him the belcantos and all the vanilla I had.