To each their own I suppose - I guess it's the same as wearing TF-related gear.
Whilst I'm a TF fan and drive Alternity Megatron's alt mode, I would never consider sticking a TF badge on it (nor would I wear TF-related clothing, ever).
The closest I came was thinking about putting a small subtle Alternity sticker on the rear windscreen, but that notion was dismissed after about 30 seconds.
Just interesting to hear people's reasons behind why they do it.
Most people I know (TF fans or not) look at cars with TF badges and snicker...