I had to replace them on my '91 - it was back in July of '09 and I had 200K on her then.
I had noted that for about a year or more prior to, my fuel mileage and performance had been dropping, especially at altitude. I recall that on a trip to Colorado, I had a hard time keeping an idle, but it wasn't a problem when I returned to Florida. When I replaced all 4 of them I noted the following in my maintenance log: Replaced Carb Slider diaphragms - # 1 & 2 were badly torn, #3 had a slight crack and #4 appeared intact.
About 18 months later, at 259K I crashed on my way to Stagecoach 2010.
I do have the carbs now off the bike, with the practically new sliders in them, along with the installed bellcrank to run a cruise control - would prolly let them go for something in the neighborhood of $350 (which is still less than what I paid for the 4 carb sliders...)