Personally, I'd trust a U-lock and chain/or cable more than a watchman. No disrespect for security personel but they can't possibly watch the bike the whole time. Thiefs have nothing better to do than just sit and watch for the watchman to leave (take a break, use the restroom or unlock a door for someone).
I used to lock my bike to the rod iron fence of the parking lot at work, right in front of the security window. The security guard made it VERY clear to me that they would watch any bikes and cars the best they can but they are not to be held responsible for anytheft or damage. I eventually found a closet in the office to keep my bike in. It pays to make nice with the building management and/or the cleaning crew.
Ernest
PS: I guess, to answer your question, it's that people don't find it financially wise to pay someone to sit and watch your bike when they can't be held responsible if something gets stolen, it may be a deterent but you're gonna use a U-lock and cable anyway and eventually ask yourself, "Why am I paying this guy?"
Last edited by Big_e; 08-21-10 at 12:56 AM.