The 0.1dB loss isn't the issue. As you stated, that's not going to be detectable by anyone without test gear.For whatever reasons there may be, I know there is some disdain for magnet mounts in some extremely qualified quarters, as evidenced by monikers forever stating: "Just drill the damn hole." There are probably bumper stickers too.
The issue is the lack of a DC ground at the antenna base. Without that, RF energy can travel back down the coax shield into the vehicle and to the radio and other systems. That can create issues in some instances.
I own two magnetic mounts. I use them for testing and temporary installs where appropriate. My preference, and what I recommend to others, is to do the installation properly, as it will prevent a lot of issues in the long run.
And as I always say, you do whatever you want, if it works for you, then that's all that matters.
I'll think about working on the bumper sticker, but I'd be concerned it would get confused with the idiotic "drill baby, drill" statement.