I've read in many places that you should change your furnace filter
regularly as a dirty filter will lower the efficiency of your furnace.

Excluding extreme cases where a filter is exceptionally dirty, how
much of a loss in efficiency are we talking about for the average
"needs to be changed in the next couple of months" filter?

PS: I'm not trying to find a reason to stop changing my furnace
filters or anything. I'm simply curious as to how much impact a dirty
filter can have. Is it 0.1 - 2% or more like 10%?

I don't have a number for you, but you may want to cosider this.

If your filter is blocked, then less air is being fed into the furnace by the
cold air return ducts. If you don't have a perfectly sealed furnace chamber,
then the furnace might be drawing some of it's air from your basement, which
might be pulling air from the first floor dwn to the basement. This might
increase the mold growth in your basement (i.e. bringing relatively warm moist
air into the cold basement).

Just a thought.