I feel the flags actually represent a negative example of using the streetpass feature (or rather, it in combination with the sidequest), but that's another argument. I also disagree with the Friend Safari example as they're locked to a FC, so as long as you know someone who had the one you wanted, you were pretty much set as you just had to get the code. That's not a luxury that the Mirage Spots have, either you or your friends have it, or they don't, that's it for the day and you're out of luck.I would agree with you on this (actually I do about the TM's since they were cool TM's), but the point of the Mirage Spots was for it to be random. It's literally in the name. Yes, it is true it's random, and somewhat reliant on Streetpassing (although not really since they change naturally every day anyways), but so are the flag captures of the Secret Bases. It's not a main component of the game, it's a random sidequest, so therefore, I think adding an element of luck is appropriate. Every game has a little gimmick that encourages going out and connecting to others and sometimes that is luck-based, but that's why they're relatively minor game parts. The same could be said for the Friend Safaris in XY. This is of course totally my opinion, I'm just ok with a luck-based system for a minor sidequest.
I will concede that some luck is fine and the Mirage Spot is a good idea in concept, but I still don't think it was handled that well in execution. It's a bit too reliant on the RNG to be as fun as it could've been as eventually you're going to get repeats rather then a new area, and adding the problems with the items did make it worse. I can see why you'd enjoy it, it's just for me it didn't work that well.