The only way I would have the creature ignore this is
a) The creature is under command from a different creature and is following orders
b) The creature has a very high intelligence and can understand when he is being tricked into fighting or not
c) The creature is unable to percieve the mark (PC is out of sight or cant be heard)
Otherwise the creature should ALWAYS follow the mark and attack it, to do something else defeats the purpose of the mark system.
As an actual
player of a Defender, I would say that you're half right. A DM should do a mix of respecting the mark, and disrespecting the mark, not one extreme or the other. If the DM
always attacks the one who marked it, then the Defender's surges and hit points become the limiting factor in an adventuring day. Doesn't matter if everyone else has 8+ surges, the Defender is down to 2 now, we have to rest because he
will go down in the next fight due to the DM never ignoring the mark.
Secondly, read the entries for Combat Challenge and Divine Challenge. If they were meant to
compel attacks, then why all that stuff about all the things they can do when a marked creature attacks an ally? Simple, it was meant to be triggered. Not
all the time, but some of the time. Otherwise, entire class features are going to waste. It would be like building an Orb Wizard and then finding out that there aren't any Save Ends effects available to Wizards. What would be the point of the feature if it never,or
very rarely, gets used?
All of this talk is good and well, but it's mostly been about generalities. How about a specific example of a power that basically becomes useless when the monster knows what it does?
Defender's Gambit - Fighter Level 15 Daily
[sblock]
Inviting attack, you lure your enemy into letting its guard down for a counterattack.
Daily
Martial,
Weapon
Standard Action Melee weapon
Target: One creature
Primary Attack: Strength vs. Will
Hit: Until the end of your turn, you gain a power bonus to melee attack rolls and melee damage rolls against the target equal to your Wisdom modifier.
Effect: The target can make a basic attack against you as an immediate reaction, and then you make a secondary attack against it as a free action.
Secondary Attack: Strength vs. AC
Hit: 5[W] + Strength modifier.
Miss: Half damage.
[/sblock]
Now, in the case of the above power, the actual hit does no damage. It only grants a power bonus to melee attack and damage rolls until the end of your turn (not next turn, but current turn). The target can make a basic attack as an Immediate Reaction, and then you get to make a 5[W] attack against it as a free action, along with those bonuses.
What incentive is there for the monster to attack? This power is granting the choice to do an extra,
special, attack as a immediate reaction. If the monster doesn't take it though, then the Fighter loses the 5[W] counter attack, as well as the bonuses to hit and damage. The monster loses nothing in that scenario though, because it was an immediate reaction basic anyway, not their normal atack. However, they pretty much completely invalidate the Fighter's whole Standard action here, unless he uses an Action Point.
Other than the occassional corner case of a really stupid monster that wants to kill the PC anyway, this power is damn near useless. A small child would see that it's a losing proposition, let alone any reasonably intelligent foe. Heck, I would even argue that most
animals, if they knew the effect here, would not take this gambit. So who is ever going to take this power?