Knife fighting is a very important skill, the knowledge of which will benefit any martial artist and fighter.  We are fortunate to live at a time when many different training tools are available for those dedicated to learning and mastering such talents.  Training knives of rubber, plastic, wood, and metal are readily available on today's market, but each has a role to which it is best suited.

Soft rubber training knives are the only knives I advocate for active sparring (with the exception of No Lie marking blades, which are specifically designed for this purpose).  Proper knife sparring requires a degree of energy and speed that makes rigid knives unsafe for this activity.  A rigid blade to the throat or even the abdomen or groin can cause significant injury.  (It goes without saying that eye protection is a must during active knife sparring.)  There are some rigid rubber knives on the market that are safer than plastic, wood, or metal trainers, but the more rigid the knife, the more critical eye protection becomes.

Cold Steel rigid rubber training knife.  It is semi-flexible.

Plastic knives are a good compromise for safety where active sparring is not an issue, such as training knife disarms and defenses.  Light but strong, such blades have the rigidity necessary to make disarms practical, but present little danger of injury while practicing counters and evasions.  The lighter weight of plastic training tools also makes them less dangerous as flying objects on those occasions when a "knife" is sent airborne during practice.

Wooden training knives are not flexible and lack the benefits of both  plastic knives (lighter weight and rounded surfaces)  and metal knives (realistic appearance). Wooden weapons are potentially lethal (and often have points much sharper than one would prefer, though some sandpaper and a file can remedy this).  I own no wooden training knives, though I do have a bokken (wooden training katana).

Edges2 aluminum trainer.  This "knife" could still hurt you.

Metal trainers are the best tools for realistic knife practice.  I do not advocate them for active sparring because they are rigid (and as strong as metal, obviously) and thus potentially very unsafe when facing another person.  They are, however, the best tools available for training forms and the mechanics of knife fighting, realistic disarm and evasion training, solo practice drills, and related activities.

Train safely and choose your tools properly for the activities in which you engage. 

Your eyesight -- and your life -- are too important to do otherwise.