When sometimes you just cant fulfill the wishes, or if there are no wishes. It would be easy to just buy a bulk of one same card, like some basic touristy card, and then just send that to all those hard ones.
When you get an address, do you look at the cards that your receiver has sent? Would you be less likely to find a perfect card, if you saw they had been sending same card over and over?
Yep! I’ve done it. Most of the time, I try to match wishes, but sometimes I am not able too. At least, that’s how it was for me when I first started. Now I have over 2000 cards to choose from. (I’ve become a bit obsessed with buying postcards!) 
Depends on their request for the “perfect” card. If they are flexible and don’t have a huge and restrictive “no” list, then what they send is to me irrelevant. But if they want only expensive cards or are picky and then they send the same thing to everyone, then no, they get something decent I think they might like, but nothing really special.
Yeah, it’s hard to hold the line isn’t it? There are so many great postcards & so many great images to make postcards with eh? I’ve only been at this 6 months but I wouldn’t need to buy any more for most of the rest of this year, lol - but will I stop?
I do look at what someone sends to get a sense of what they like to send, especially if there are fewer clues in their profile about what to send.
I’m fine with people sending the same card over again - the reality is, if you’re trying to do Postcrossing on a budget & many people are - you can get really good prices for bulk buys with printers - like 50 copies for $10 so I can see why some folks do this.
Also if you don’t want to spend a lot of time choosing cards or don’t have the time or don’t have lots of cards, it certainly simplifies the process and you can just concentrate on the message.