Using a >100% scale factor on X11 is unpleasant without Qt scaling.
Icons, checkboxes, and radio buttons are too small, the panel is too skinny and must be manually adjusted, text in buttons feels squeezed, and so on. The size relationships between items is broken and overall everything just feels... off.
Using Qt scaling resolves all of these issues, at the cost of blurry icons for people who are using fractional scaling with low or medium DPI screens. However these people already put up with this in their apps (which do respect Qt scaling) so they are signaling with their choice to do so that they're okay with this trade-off. Plasma currently does not honor the preference embodied in that choice; this commit makes it do so by turning on Qt scaling.
For people with high DPI screens (1440p or 4K) using a 200% or greater scale factor, the pixel density of their hardware is sufficient to keep icons looking good, so there is no trade-off to be made and the result is visually better in every way.
In the past this was not done due to various blockers caused by mixing screen space geometries (scaled vs unscaled) between Plasma and KWin, but all of those blockers have since been resolved; see the "Depends on" list in https://bugs.kde.org/show_bug.cgi?id=356446 for details.
I have been using Qt scaling in Plasma for over a year, both with a 4k screen at 250% scale and a FHD screen with 125% scale and in my opinion the results are overall visually superior to the status quo.
BUG: 356446 FIXED-IN: 5.20