I have gone as much as 20 years without a single stage press before so I wouldn't say they are needed. I actually prefer turret presses myself to a SS most of the time but there is not much that one can do on a SS that couldn't also be done on a 550.
Single stages are not needed if you don't load for accuracy. For sure. But need is a blanket statement.
I said you needed one for sizing bullets. Well i have sized with a Lee sizer die on a progressive. One bullet and i knew how bad an idea it was. I had to make an adapter just to do it.
Do you need a single stage for it? No. But its more trouble and it was made for a single stage press to begin with. The forces required to size a bullet are sometimes a lot and doing so on one side of a progressive or turret press can break or damage it. Especially a load master or pro1000. Don't know how many you have sized in your 20+ years but if you did them on a progressive you did them the hard way and maybe you didn't need a single stage press but you would have been much better of with one for bullet sizing.
Why i say a single stage better for accurate rifle loads:
Fact: Almost all bench rest shooters use a single stage other than bill whidden ( cut) ( fact 2) and unless your using his tool head and still loading charges by hand your not going to get that same consistency in charges or OAL for precision loads . Two of the biggest factors in the reloading process ( not case prep mind you) to insure accuracy and both are affected negatively by progressive reloading alone.
Now I'm sure some do fine reloading rifle rounds on a progressive for plinking and can manage average accuracy loads on them too. Heck i wouldn't sit on a single stage and stamp out a 1000 .223 loads for plinking. I would use a my progressive press for that.
Just to add there is the shoulder bump ( another process more accurate on a single stage) when Fl sizing. You may not do it and many do but if you get that wrong and bump too far you are causing the brass to be worked and stretched more than what is necessary in a place where the metal is not as soft. That weakens brass at the shoulder and can lead to early case failure.
But if your loading for different rifles of same caliber using one load and not loading for each rifle individually then your not loading for precision or accuracy as a result. So again, not needed but your not going to accomplish certain quality loads without certain changes. So for those that do seek that they have a good use for a single stage press.