I've been burned by PuGs, and even the occasional greedy kid within my own guild who /needs an item they would rarely use, yet would be a primary item for me. I'd hate to run an instance to find something I need, only to have someone doing 5% of the work end up rolling a 97 for the loot. Unless I'm helping a friend, I'm not running anyone through an instance unless it's an easy and quick run.