I regularly lock my good quality folder up outside - including outside classroom buildings at the nearby campus. Depending on where it is I use either a cheap cable lock and/or a better U-lock and haven't had any problems in the 16 years that I've had the bike. My impression is that most bike thieves are looking for something with a known high resale value that they can resell easily. Folders are relatively uncommon so they 1) don't know what different makes/models are worth, and 2) they might be worried that the stolen bike would be more easily identified when they try to sell it.

When our daughter went away to college we heard lots of stories about the high theft rate of bikes on her campus. But a walk around the area revealed lots of bikes with either very cheap locks or that were just locked to themselves rather than a fixed object. We figured she just needed to make her bike less attractive to a thief than other bikes there and got her both a cable and a U-lock. Except when being ridden, the bike was locked outside 24/7 during her time at college and now, years after graduation, she still has the same bike.

Obviously there are no guarantees, but my feeling is that with a good locking strategy your bike is unlikely to be stolen.