> Great! Then the user gets his preferred font, as requested, instead of the one the page specified.

No. You've misread the main point. The user would have gotten his preferred font if the font stack was either just plain

    font-family: monospace;

or

    font-family: <list of fonts their system does *not* support or does *not* allow to be used>, monospace;

. But the case is that the suggested font stack contains some "unwanted" font that their system both supports and allows to be used, that precedes the generic `monospace` font family the user actually prefers, or, more precisely, have assigned their typeface to. Is it more clear now?

I agree it is not a huge "bug" on the first sight, and as it seems even this is somewhat solvable without disabling font support completely. But since it takes some effort and expertise on the user's side, it adds the "bug" some weight nonetheless.