I'm a huge Louis CK fan. Here he on David Letterman addressing this issue: when your kids wants to have or wants to do what everyone has or does!
Sometimes it is completely OK to cave. I once coached a parent who had a no TV during the week rule. This was back a few years when the show "Greys Anatomy" was a new show. It seems that all the girls in this particular middle school were completely hooked on this show. Friday lunch talk after the Thursday night episode was these girls "water cooler" moment when they obsessively talked about "Mcdreamy" and all the other characters and their lives. This mom's daughter was completely out of the loop, and began to dread going to school on Fridays. ( I know this seems ridiculous, but being part of the group is about as important as it gets for middle-schoolers) This mom was holding firm in the no TV rule, and her daughter was furious. "How come it's OK with all the other parents but not with you?" And then came the TV is a distraction, stupid shows lecture. Which completely fell on deaf ears. I asked the mom, truly, what would be the big deal if her daughter watched this one show a week. If she could show that her homework was done, couldn't they make this an experience they could enjoy together. Mom thought about it, and realized that the good out weighed the bad, and caved.
So sometimes the bigger picture (this girl could feel a part of this group, and mom and daughter had a special night during the week when they shared something that was important to the the daughter.) These are conditions for a good cave.
But a bad cave, that's a different story. Are you being asked to allow your teen to do something you know is unsafe or unreasonable, and cave just because you don't like conflict, or you don't want your teen to be mad at you. This is an unhealthy cave. It gives your teen power and precedent. Not setting limits on cellphones and computers because your teen doesn't want you to is a bad cave. Allowing your teen to go somewhere that is unsupervised and potentially dangerous and unsafe is a bad cave.
Its important to know the difference!