From experience, and from being on the other side and seeing it personally, I know that when someone is sick or really anticipating a shot of heroin, they get anxious and shaky as hell. I've had people come over to shoot up, repeatedly shooting themselves in the same spot, shaking like mad, and still refusing my help. Then, 20 minutes later, when their arm looks like a bloodbath and their rig is almost clogged, they come to me. I make them tie off, cautiously look around for a better vein, and always end up hitting them in one try. Given, most of these people were rookies, but still, the crook of your arm definitely isn't the only place you can shoot, and it is also a place where your track marks are going to be more noticeable and harder to go away. I know people that would rather tear the crook of their arm up to shreds than use their wrist/arm because they don't want mommy/daddy seeing a little dot on them. What happens when it's hot as fuck outside and your parents keep asking why you are wearing a damn sweater? Some people man, some people
As for me, mine were all blown to shreds for a while and I ended up getting some pretty bad marks on my hand, but things are coming back and I keep finding new spots that are working again. Thank bejesus for fresh rigs!