I'll take a stab at this. If you have good R8 collets, like Hardinge collets that have a confirmed run out of 0.0002", then you are contending with the fact that R8 collets grab the tool shank with only about a 1/2" of surface area, much like a C5 collet, and the fact that the tool shank must be within about 0.005" of the collet size to be held solidly. Good R8 collets are quite accurate; junk collets are not.
An ER collet system has several advantages. It grabs the tool over a longer surface area so it registers the tool more accurately inside the collet. Due to the geometry of the ER design, it surrounds and clamps the tool shank very solidly. If you have an accurate chuck and use good collets then run out is minimized. Moreover, an ER system dampens vibration better due to the mass of the chuck/collet/nut system. Finally, a good ER system is very accurate over a wider range if the chuck is good, the collets are of good quality and you use a good nut that is torqued properly. Granted, the collet is most accurate when used with a tool shank very close to its stated size but small variations in tool shank size can be easily accommodated.
One of the key reasons that ER collets are a standard in the industry is because rather than change the tool in the chuck, the entire chuck is changed when a tool change is needed. Tools are locked into the chuck and remain there unless it wears. Imagine if you had to completely unscrew a draw bar, change out an R8 collet, change out the tool and then screw in the drawbar again to lock the new tool in. Very cumbersome.
I use the Tormach TTS system and have several ER chucks. It is very fast to swap out one ER chuck for another when multiple tools are needed. I think you can go either way in a hobby shop but for me, the ER system has enough advantages that I prefer it over R8.