I always carry a knife both for defense & as an offten used tool.
Although, I seriously believe that you should take some knife fighting training if you plan to carry a knife as a defensive weapon.
If you pull a knife & your opponent chooses to engage (with or without a knife of his own) things will get up-close & very, very messy quickly. Plus if he knows what he's doing or just gets lucky you'll end-up on the receiving end.
Obviously, if you have had any training just ignore the above.