Ask "What is the purpose of the knife, can a guard enhance its use in purpose?"
Guards do two things; prevent the user's hand from sliding onto the blade, and/or prevent something else from sliding up the blade onto the user's hand.
A utility knife generally used for light cutting or slicing likely won't need a guard. There the force of use is towards the handle, a pulling motion is used, unlikely the hand will slide forward onto the blade.
Any knife designed for forward movement, however, perhaps for stabbing or thrusting, or which might be used for defense, should in my opinion have a guard. The guard on such a knife will prevent the user's hand from sliding forward onto the blade if the point hits something hard (bone or armor), or it can be used to parry another's blade, protecting the hand.
If you have ever fenced, then you can understand how important a guard can be for protecting the hand from an opponant's blows!
Andy