Having a high kdr might be nice to boost your ego but end of the day i'd rather have a bad kdr and just enjoy the game rather than going nuts over a bunch of stats.
And there's lots of time it makes sense to finish off an enemy with one or two quick bullet then res, there's always a chance you that you will get killed while you proceed to rez your teammate because let's face it, you're a sitting duck during the rez.
I was killed while doing it and sure as hell will be again.
I sometimes leave executions for the teamates who are struggling. Wouldn't it make more sense for them to get the parts they need to upgrade their weapons or armor and be more of a benefit to the team? That way the whole team can perform a bit better and everyone ends up doing better. Plus a lot of people need executions for missions, though I personally never choose that option as it seems less productive.
I always go for revives over executions unless there is an enemy nearby that might take me out if I revive and will revive their mate if I flee. At that point my downed team mate is dead either way, might as well make it an even split.