I'm a bike geek and love the freak bikes, so I am naturally attracted to folder. Just didn't really have an excuse for one until recently.
I try as much as possible to bike commute. For a while, that had been a 19mi ride o/w on rural and urban streets, and bike of choice was 26"-wheeled traditional dd frame dedicated commuter.
New work situation is 45 mi away and while I do want to do that commute a few times, not on a regular basis, so commute is now bike-bus-bike. If I want to stick with commuting, I either park a beater bike at the work end, or bring a folder.
As it turns out, through the winter, I am keeping a beater locked up at the work end bus stop. Part of the downside of folder ownership is proprietary or non-standard parts. No studded tires in 18" size for my Birdy, so the beater has studs, so does my commuter.
I don't need a folder... but I like it a lot better than the other option and will stick with a folder. No worrying about a bike locked far away, even a beater I won't mind losing. No worrying about an uber-commuter bike locked at a bus stop when I'm at work. No fiddling with locks or leaving the bike out in the rain.
Recently, I had to go to San Francisco on business. Birdy packs down to fit into a standard Samsonite suitcase I already had. Got a hotel a few more miles away from the office for cheaper money and rode every day to work on a familiar bike. No hassles with rental bike in who knows what state of tune, or parking it outside.
Folders are fun casual and commuter bikes. Others use them for much larger rides. I don't see a lot of downsides to them, and am only finding more and more uses where the folder works where a full-size bike wouldn't. Stuffing it in a car is the easiest thing, bus drivers don't mind folders like they do full-size bikes, travels with me on a plane w/o hefty oversize/bike surcharge.
And everyone loves them, thinks they are cute.
So while I still don't "need" a folder, I don't think I'll be without one in the stable for a while.