SIZE
I'd much rather carry a larger caliber. (I'm not so sure I'd want to shoot a bigger caliber in the exact size gun as my LCP very often however.)
I carry two J-frame .357's when I can. One of them is a 340M&P. I love it. But, no matter what they say, it just isn't small enough and comfortable enough for my occasional front pocket carry needs. I use it in coat pockets and as my back up in a belly band, off side shoulder holster and in bigger pockets in general.
I tried PM9's and the difference in size vs. my 340 wasn't enough to warrant getting one. It was drop back to the LCP or stay with the 340.
Like I said, I love the 340. But if it gets left home when wearing jeans, shorts and such it's not much good to love it - is it?
Little guns like the LCP serve a purpose at times. Most serious carry guys will choose something hotter if we can though IMO.
I do have a concern that the legion of little .380 carryers out there don't understand it's limitations enough.