based on Anorak's answer, I also agree with Zorayr's concern, so I added a couple of lines to remove the UILabel and return only the CGFloat, I don't know if it helps since the original code doesn't add the UIabel, but it doesn't throw error, so I'm using the code below:
func heightForView(text:String, font:UIFont, width:CGFloat) -> CGFloat{
var currHeight:CGFloat!
let label:UILabel = UILabel(frame: CGRectMake(0, 0, width, CGFloat.max))
label.numberOfLines = 0
label.lineBreakMode = NSLineBreakMode.ByWordWrapping
label.font = font
label.text = text
label.sizeToFit()
currHeight = label.frame.height
label.removeFromSuperview()
return currHeight
}