One more gear, just as durable/reliable (really... assuming the one-time use pin is installed correctly or a reuseable link is used), NOS and OEM parts are still available today (stock up) and wholesalers like QBP will continue to source aftermarket replacements sold under generic names for 8 and 9 speed so long as there is a demand. 8 speed is a bit cheaper.

I switched from 8 to 9 speed back in 1999 and never changed my cranks/chainrings, it's only when you get to 10 speed that chains can become narrow enough to drop into the gaps between older cranks.

As far as the value of adding another gear, we alternate between 9 and 10 speed these days and the way we ride I like tightly-spaced gearing and I really can't tell the difference between a 9 and 10 speed 12x27t cassette, but can tell the difference when we use a Campy 13x29t 10 speed... because we get a bonus gear that we can actually use: the 29t.