Another thing to note about these marking powers (relevant to having multiple characters, rather than the multiclass issue at hand):
A creature can only be marked by one character at a time.
That means that if a paladin marks things, the fighter has no longer marked them. More importantly, there are a number of powers that other classes have which mark things--those classes not being typical defenders. One thing this means is that those can be used effectively for multiclassing in some ways. The other thing it means is that those other classes can act as mini-defenders. And the final thing is that they are making themselves vulnerable to that enemy if a defender previously had that enemy marked.
(Specifically: Cleric's "Healing Strike", Pathfinder's "Slasher's Mark", Knight Commander's "Break Their Nerve" and "Control the Field".)
The Knight Commander's abilities are clearly intended to let you act as a defender (and are very appropriate for paladin or warrior multiclassers.) The other two are interesting.
The Cleric's ability is potentially dangerous: It breaks the defenders' marks and makes the Cleric more vulnerable to attack if he uses that power. But it also could be used to "defend" the target of healing--you go heal the wizard with it, and you mark the enemy that got close, and now the wizard's probably not going to hit again. You've bought time for the defender to get there and save the day, or for you and the wizard to deal with the problem. Finally, it's a drawback to a fairly powerful low-level power.
The Slasher's Mark ability, on the other hand, is very very interesting. This seems to me to be intended as a "defend by running away" sort of maneuver. It marks until the end of the encounter, which is huge. You've just spat in the face of whichever things you attacked, and can harry them and hit and run while they chase after you (or on the other side, they can ignore you and suck up the -2 to hit.) The chase seems appropriate given the path's description.
Interesting stuff, all around.