Meatpuppet said:
Wouldn't a +5 bonus to morale save be more appropriate in these situations?
I don't know maybe I'm wrong but a creature, even if attacked in its lair, could be tempted to take a hike (against ovewhelming odds that is)
The key is "take a hike" to where? If the monster is already in the place where it would go under other circumstances, the check is pointless.
However, I should point out that the Morale checks in Heroes of Battle are much more significant than those used in previous editions.
Previous versions had results of "stay and fight" or "run" only. In the progression outlined in Heroes of Battle, a wider range of effects can result (Shaken, Cowering, Nauseated, and so on). Many of those effects were previously generally available by the use of spells but not often by other means.
I am actually pleased to see that appear, both because I disliked the implied lack of a retreat option for monsters (although, as other posters have mentioned, playing the monsters based on their motivations is better); and because I like seeing the range of options other than "stay" or "go". Under the Heroes of Battle setup, a monster trapped in its lair could be forced to stay, but could also fail such a morale check and be "Shaken" while fighting.
However, I was paraphrasing the advice in Heroes of Battle (see the Morale in the Dungeon section of the Morale rules) when I said that sometimes, in the dungeon, the morale check is inappropriate because the monster is "in its lair". It is up to the DM to decide whether a given monster "doesn't need to make a check" or, as you suggested, "gets a +/-x modifier". A mindless undead set to guard a tomb, for example, probably doesn't even need to make a check at any point; a single living warrior 1 kobold facing a party of adventurers probably has a -2 (or worse) modifier when cornered; a single living warrior 1 hobgoblin might get a +2 bonus (instead) when cornered.