True. When I start doing instances/raids, I will only need roll for something that I could use and will use. Other than that, I just pass.
Why not greed? If nobody needs it then you might as well have a chance to get it and sell it.
Anyway, it doesn't really matter in raids as most people use the Master Looter system, with only the people who can use it for their main talent spec are allowed to roll. The loot master then decides who gets it.
The loot system will never be perfect, but I see no reason to change it, there are bigger issues to handle really. Sure, the system can be exploited, but that's a risk worth taking in my opinion. Besides, if you notice someone is a ninja early on you can always just kick him, or switch to master looter to make sure it doesn't happen again.
I'm not sure if it's like this on every server, but on mine, if you ninja something in a raid or the likes you will get your name posted in the trade channel most of the time, which makes it difficult to find new groups. Especially when everyone involved tells their guilds and suddenly 10 or 20 guilds want nothing to do with you.