It's just a little messed up, because this way you have two defaults. Default in traditional sense (= what's used when it's not specifically set in ini) is still (and will always have to be) Microsoft Sans Serif and new default is just a special case saved to clean ini. It seems strange to me.
I still think the problem is mainly because of using specific font and would be much smaller if system font was used. Segoe UI Semibold is clearly wrong for every single user of Classic theme (except for maybe one or two who actually selected this font for their system :). It's of course possible that some Aero user liked Microsoft Sans Serif in TC, but my guess is that they should not be hurt too much by same font they have everywhere else. No hard data to prove it, unfortunately.
I guess there are some misunderstandings here. It works perfectly as Christian mentioned. If you are updating from the beta 5 you will get the old font back. However if you do a clean install you will get the new font. Pretty sure that's how he intended to work. If you deleted your ini file there's no [Configuration] section so Total Commander will write the UseNewDefFont=1.
Anyway I think that the new font looks much nicer. Got used to it since the beta 5, and I'm not going to change it back. For those who don't like it, there's always the option to revert back.
Now try doing the following. Launch Total Commander with a clean ini file, then close it. Edit wincmd.ini and remove the UseNewDefFont=1 line. Save it and relaunch totalcmd. You will see what happens.
The reason is that "Microsoft Sans Serif" non-bold looks quite OK in the dialog boxes. I do not want to switch from non-bold to semibold there, and switching to another non-bold font doesn't mak that much of a difference.