Of course I do, for piece of mind. But if you think locking your car or house will keep out a determined thief, then you are crazy.
And if you think that a Microstock Agency putting a watermark over your image is going to keep a determined thief from using it, you're even more daft.
Of course a determined thief can get anything, even if it's locked up in Fort Knox. Nothing will stop a determined thief - certainly not a water mark on an image on the web. But ...
a) It will mean the thief has to do a bit more work
b) You will have something in the way of proof that a theft has taken place.
If you reported a burglary to the police and then told them you there was no sign of forced entry because left your house unlocked "...
a determined thief could get in anyway ..." , what do you think their reaction would be?