I don't, it's available so I read it. I provide comments on it. I participate in the community that is an open source project. Could you hide malicious code in plain sight? Sure. Would the project see the light of day when said malicious code was discovered by someone with knowledge and the time to read the code? Nope. I'll use an open source project over a closed source one in a heart beat for that reason. The likelihood something is hidden, and undiscovered, is much lower.
I am not sure what you mean about the project seeing the light of day, though I commend you for actually working your way through the code. You represent a tiny minority of computer users however and AllBitcoin is clearly not for you. OSS has many fantastic properties and I'm a big supporter of it. Many eyeballs on the code are great for security and bug finding. However OSS does not imply trust. For your amusement I recommend you check out some of the code presented here: http://underhanded.xcott.com/
Most of us trust banks, OS vendors, video game developers, etc. enough to use their closed source software. I would prefer for it to be open sourced too, but that would not influence my level of trust in these organizations.
Bitcoins have helped expose how lax our security has been and if we want it to succeed, we have a lot of work ahead of us.
And how did Bitcoins expose lax security? Because users had trojans? Because a site got hacked? What does any of that have to do with Bitcoins? We don't need new banks because someone hacked a bank.
I meant it in the sense that prior to Bitcoins, most losses due to compromised computers could generally be reversed. Bitcoins give us irreversible transactions and a store of wealth sitting directly on our hard drives. We're not quite ready for this in terms of both social and software engineering but I'm sure we'll figure it out.