Some folders would work quite well as commuters, and there are several that ride like regular bikes. However, they are about twice as expensive, give or take - for $450 you can buy a 24/27 speed hybrid that would work well for most commutes. A $500 folder will only have 7/8 gears, however; to get a folder with 24/27 gears, you need to pay around $800 to $900. Or much more.
The $900 folder may well have nicer components than the $450 hybrid...but you have to pay that much just to get into the game.
Note, too, that folder that ride more like regular bikes tend not to fold as small as other folders (tending to have 20" wheels, for example), and once you equip them as a typical commuter - with a rack that will hold full size panniers - the fold is even larger.
Having said that, I do commute on a folder, and it works very well. But I bought it to take touring, not as a commuter, and I don't fold it as part of my commute, as I typically put it in a bike locker at work or just bring it into my office. On a couple of occasions I have put it in the trunk of a coworker's car when we were going somewhere after work, but this could have been done - with a little more difficulty - with a regular bike.