I'm enjoying the thread! Is it "annoying" and "distracting" because it pulls one out of the so-called 'suspension of disbelief' necessary to enjoying fiction, esp. fantasy fiction? i.e. it reminds one that these are actors rather than the characters they play? BTW, I hadn't realized the Tom Lenk one, either. So we've got:
Jasmine/Zoe -- Gina Torres
Holden/Knox/Tracey -- Jonathan Woodward
Caleb/Mal -- Nathan Fillion
Hamilton/Jayne -- Adam Baldwin
Cyrus (vamp Lenk)/Andrew -- Tom Lenk
Kane (werewolf hunter)/Sahjhan -- Jack Conley
Ken/Alliance agent Dobson/Richard Straley -- Carlos Jacott
Are there more? I think CowboyGuy is right, too -- I think Whedon tries to reuse actors who have worked well for him. The idea that makeup and character matters is an important one, too: we notice the recycling when they are most similar to the other characters they played, which suggests some typage or typecasting. Hmm....