Staying on the dreaded side lock endmill holder theme, there is a significant quality gap, though this was not a comprehensive test. We measured two 3/4" stub length toolholders from two suppliers with a 3-point bore gage. After calibrating off of a ring gage, the first bore measured 0.751" and the second 0.75005". We also measured ten random 3/4" carbide blanks (as we did in the earlier post with 1/2" blanks). We then loaded each of the ten blanks in each holder (tightening the screw on the round blanks, no flats) and measured the total indicator runout (TIR) in a high end tool presetter. Toolholder #1 ranged 0.0011" to 0.0012" in TIR and Toolholder #2 from .00020" to 0.00030".
Side lock toolholders are still in use because, they still work in some applications. As we pointed out in an earlier post: https://goo.gl/gEaNuP
by design endmills are trying to twist and pullout of toolholders. Many other toolholders are held by friction only. Think of it this way; if you wanted to drive a gear or a sprocket from a motor shaft, would you rely on a press fit? You would use a key and a screw (or a spline). Dynamically, side lock holders can be made very short (1.75" gage length minimum) with significant damping.
Draw your own conclusions.
Sharing information about high performance milling technologies, the result of 30 years of research.