Yeah, that won't work so well with the phone in a pocket since the mic on will be substantially covered.
In a car (if you're driving) you wouldn't want to use a headset anyway, so having one with a mic for that is really a very different situation. Some people like to use higher quality headphones or earbuds, and most often those will be wired. So, having a working mic on the headphones is nice, especially if you're moving around while talking and don't want to worry about pulling your phone out of a pocket.