At least one compelling reason to run a chainguard is to keep your chain clean. If you are the sort who hates the onerous chore of chain servicing, then it is only a matter of time before you succumb to the ultimate cycling insult - neglected chain 'greasy leg'.
By keeping the grit and dirt off your chain with a guard, you can cut out half the worry of chain rub.

For Danish babes hauling around Copenhagen on upright three-speeds, chain guards are also the ticket.



See? THAT works, doesn't it?

Ditto, if you wear a properly tailored suit while commuting. Keeping your $350 Brooks Bros. gabardines out of the chain on windy days is a good thing.

Besides, everything I can find includes the chain guard as standard equipment on bona-fide commuter bikes. After all, if it's on the internet it must be right... right?

Finally, chainguards are the ultimate in nerdy chic (which is their main detraction here in the USA). At least, that is the case for the full coverage guards.**
If one is compelled by either actual lifestyle or internet 'bike rules' to look completely dweebish, nothing else will do it like a chain guard.

Think 'Dilbert GoesBike Commuting' and you get the idea.

** The Chainboard guards actually have a hi-tech look and an appeal that works. At about $25 all in, I'm gonna get me one.