No, I kid ye not.
The Mysterious Stranger would always appear at the end of a VATS sequence, which I'd say by that time (most of the time anyway) the enemy you're shooting at is either dead from VATS or near dead from VATS, even more so if you have high Luck in the first place considering the critical hit chance you have anyway. At most, the only thing the Mysterious Stranger is doing at that point is just finishing off something you could have already done yourself outside of VATS. The only exception I see for this is for one of Broken Steel or Mothership Zeta's absurd bullet sponge enemies, when a single VATS isn't going to cut it for them and the Mysterious Stranger's damage could be helpful (but then it's just a perk that is only good to deal with bad game design
). But of course, that's not taking into consideration IF he shows up. I remember playing a high Luck playthrough when taking the Mysterious Stranger perk and out of 200 hours, he had only appeared 32 times in VATS. No doubt a lot of those times were most certainly ones I didn't need him to finish off that measly raider that was one shot away from death, or whatever other time he would bug the hell out of VATS and leave my character standing there getting shot at because I'm stuck in the VATS sequence. So yeah, quite useless, and not one worth getting in my opinion.
Now, you're right about one thing. Luck can be a powerful stat. But like I said, if you have any interest in dealing critical damage that is. Very powerful in that regard. But in its other areas regarding looting, like if someone was going to invest in Luck in Fallout 4 for looting, I'd say not very essential based on experience with previous games. But you're right about me not knowing the specifics behind the numbers for the stat. All I know is from my experience I felt no difference in the loot I was getting with high luck compared to low luck from my many playthroughs in the Fallout games (the only exception being the random encounters which I noticed appeared far more often with higher luck, and some of those encounters have some loot behind them). By the end of it you're going to be inevitably well-equipped regardless. But if you know the numbers, by all means, do educate me on it and I may just have a change of thought.