It's not trust or distrust, it's the complete absence of any attempt to find out more about how these exchanges/banks/wallets, operate.
For fuck's sake, there should be a fucking mob of people with lit torches hounding the exchanges about whether or not they speculate on their own exchanges, the real state of darkpools, a written roll back policy, written policies on how YOUR bitcoins are being held (can they be invested etc, who is overseeing it?, if they are being used for business expenses, how much? If they are being invested, how so?). But no, no one asks anything, and anyone who suggests that they shouldn't be trusted gets labeled a troll.
I wrote
this post a few weeks ago about what us former poker players learned about money + limited regulation + interwebs. It spells trouble way more than not. The first poster was Tradehill's owner agreeing that we should be putting the screws to Exchanges, demanding transparency whenever possible.
You should be VERY concerned about a "bank run" on various BTC exchanges. I would bet an appendage of your choosing that some of these exchanges are not keeping 100% of bitcoin deposits in earnest. That they are liquidating them to pay for business related costs, optimistically, and less optimistically, they are cashing them out for hookers and blow.
You should be VERY concerned about having money on an exchange where the amount of bitcoins being held far exceeds their profit (a lot of exchanges have sub 100 btc volume per day). The only exchange that would have a problem walking away with all of their bitcoin deposits would be MtGox simply because they are the largest. If I'm sitting on $500k USD of bitcoins, and my site is only making $100/day profit, you can't fucking tell me that there is no temptation to walk off. Given the steady volume decline of the last few weeks we are in this situation for many exchanges, and if not, we'll be there soon.
Stop being fucking naive and drop the rose tinted glasses. Just because you're asking hard questions and are skeptical, doesn't mean you are anti-bitcoin.