We really need to differentiate between the two types of "security" when we're talking about them. Are you worried about your personal information being stolen? That's security. Are you worrying about being spied on? That's privacy. People generally aren't worried that the NSA will steal their personal information, run up their credit card bill, etc. They're worried that the NSA will see something that could be used against them in court, or used to target government actions against them in any way. Not to say that an NSA backdoor couldn't compromise both security and privacy, but this is a simplified view.

BitLocker is secure in that it keeps out the attackers. If it keeps out the NSA is a different story (one that is much harder to determine).