Right. Being the person who made the claim that a baseball bat attack is more potentially a lethal attack in the first place, I think it might be pertinent to explain what that was in reference to, as it wasn't to the amount of damage that each weapon could potentially cause. It was more to do with the more commonly encountered tactics that you might come up against.
When it comes to a knife, the most common usage is to threaten/intimidate, commonly in order to extort compliance or money out of someone. The next is a "defensive offence", in which the knife is used to create a barrier by slashing back and forth. The idea is that the person doesn't really want to do much damage, or kill you, but is using the knife to maintain a sense of power. Finally you have the dedicated attempt on your life, which is far more commonly a stabbing action than a slashing one. While common in prisons (where a lot of people seem to get their ideas on knife combat/assault from), it's not as common as a "street" assault, to the point where it makes up the minority of knife attacks.
With a baseball bat, it again can be used to intimidate, either by showing it and waving it around, or by hitting other objects (think road rage incidents, where the car becomes the "victim" of the attack). Once it becomes a case of actually attacking the other person, whether it's the "defensively offensive" actions, or an actually intended lethal assault, the common targeting is the same: the head. That's due to a range of reasons, including the psychological aspects of attacking the "face" (which represents the person, psychologically speaking), as well as it being perceived (accurately) as a powerful action. In other words, as soon as the baseball bat-attacker moves past intimidating, the most common attack is going to be potentially lethal, whereas with a knife it might not be.
So the reasoning behind saying that a baseball bat attack is more likely to be potentially a lethal attack is based on the type of attack more prevalent with a baseball bat, not on the amount of damage that could be done, or the type of attack that could be made.
The thing is that aiming at the head is the more common method for a baseball bat, not the arms. And as for the effects of the slashes, as seen in the well-worn photos, well... they didn't die. Most of those wounds are nasty looking, but realistically superficial. A baseball bat to the skull is not really such a superficial injury.