"...but there won't be a 9mm as small as a Keltec P3AT."
That is the biggest "pro." I suppose there could be 22's and 25's that small. The Kel-Tec 32 is a tiny bit lighter, at least. But the P3AT/LCP type of 380 is the smallest, lightest gun that you can get in a reasonable SD caliber. I would rather carry my 44 snub or 38 snub or "pocket 9", but I can carry my P3AT when none of my other CC guns would be anywhere near as comfortable and convenient. Any weather, any clothing, I can always have it with me.
I have had it for five years, shot hundreds of rounds through it, and have never had a malfunction (knock on wood).
Where I live the legal "reasonable distance" for SD under most circumstances is 20', right around seven yards. I can hit a paper plate with my P3AT every time at that distance. It would be FUN to be able to shoot it well at 15 or 25 yards, but it is not necessary. It is a close range personal defense pistol, not a target pistol.
IMHO the biggest "con" is that due to their weight and size they are unpleasant to shoot. I know that if I put 50 rounds through mine, my gigantic hands will be a bit roughed up and sore afterwards. The extended magazine helps a bit and I do use it at the range some of the time.
Like everything else, it's a trade-off. You get a SD pistol in an okay (but not great) SD caliber. The only other handguns as small and light are not acceptable to most people due to caliber. It is easily concealed and comfortable even in shorts and t-shirt.But it is a short-range weapon that is difficult to shoot well past 7 yards or so. Its caliber is the weakest caliber that many shooters would consider for SD. It is not fun to shoot. Its capacity is limited.
If you think of it as an OPTION, it is a good thing to have in the rotation. But I am happy that I have other options too.