Mal Malenkirk said:You'd have to be pretty ingeneous to come up with a scenario where that matters...
Your first mark isn't going to get a chance to attack somebody else before the end of your turn.
It's the end of your next turn, so it can come up in rather a lot of situations. Let's say that we have two monsters, M1 and M2, and 5 PCs, A, B, C, D, and E. The initiative order goes:
M1
A (Fighter)
B
M2
C
D
E
On his turn, the fighter attacks M1 and marks it. On M2's turn, it tries to move past the fighter, provoking an OA. The fighter swings and marks M2. Now M2 suffers a -2 to hit anybody other than A with its attack, and C, D, and E have much less to fear from OAs against either monster--and if the monsters are still adjacent to the fighter, said OAs provoke a free melee basic attack from the fighter. On M1's turn at the top of the next round, it still suffers the penalties of being marked. Then A's turn comes around again, and at the end of it the marked condition vanishes on both enemies.
Note that the same scenario applies if A uses a power to attack both M1 and M2 on his turn, with the only difference being that in this scenario, PC B has less to worry about from OAs.