I grinded Con up to 34 for the skill before I lvled my PLD. It's a great skill to have even if you lvl up other classes. I used it heavily in my pre 50 dungeons since it gave me a couple of free hits. Especially if I didn't know what all the abilities were of the bosses I was fighting. Doesn't take too long to get to 34 with fate grinding, so its definitely worth the investment.
As for mercy stroke, I don't have that one. But as many other people have already brought up its downside, personally I probably won't go for that one unless I'm really bored.
