Locking zippers won't keep out a determined thief, but they don't even need to do the pen-and-zipper trick -- they can just cut the fabric with a knife. However, many acts of thievery are opportunistic. People who wouldn't normally commit such acts see an easy opportunity and, maybe just for dumb kicks, perhaps because they're drunk, decide to take advantage of it. This can be as simple as a fellow backpacker, drunk from a night out, deciding to take a look inside your bag for a laugh, just because they can.
So, while a locked zipper on your bag won't stop a serious thief (and given this, you shouldn't keep anything truly valuable inside it), it is
a deterrent against opportunistic thief.
I do agree that the huge PacSafe backpack cages are not worth it. As said, they only draw attention to your bag, but more importantly for me, they're just too damned heavy. However, I do personally use and often recommend using the smaller PacSafe DaySafe bags for a laptop (see my post on how to lock a laptop
) and other valuables if
you don't have a safe to lock them in (or if you don't trust the safe). Again, they may not stop a determined thief, but they'll do a lot to stop anyone who's not expecting it.