D'karr
Adventurer
I agree. An obvious weak-spot should be exploitable, but it shouldn't be absolute. In the d20 system, I'd say that swings of up to 5 in either direction are about the limit. So a monster might have 5-higher than baseline in one defense, and 5 lower in it's worst, for instance, and still be usable.
Instead of giving the base monster the weakness, give the DM information to make that call. The +2 /-2 (DM's best friend) can be adjusted to size for whatever situation the DM prefers.
In the case of the Grell, as an example, the DM could have assigned a bonus +2, but given a bonus +4 if attacking from above.
These type of things are better, IMO, when given to the DM as guidelines for running all sorts of situations. Instead of "hardcoding" it to only the situation with one creature by putting it in the stat block. YMMV.