It's funny, same letter names are not my style, and yet I have two Cs, and would consider a third (although really hoping to not go that route). It was most important to me that I LOVED what I was naming my kids, and then everything else was secondary. DS is named after FIL, so there wasn't a whole lot of negotiation there. And DH and I just adore DD's name, and couldn't come up with anything else we liked as much. At the end of the day, for some reason we're really drawn to C names!
I also don't even bat an eye when I hear of other people with three children with the same first letter names, but when I'm talking/thinking about my own kids, it feels funny in my mouth to say it. For me personally, I dislike rhyming/themey sounding names and alliteration names (first and last initial the same) much more than same letter names.
I think the Duggars really have done a disservice to this trend/theme.