I understand older-editions morale rules, but they weren't used frequently in AD&D (they weren't in the MM, nor included in monster entries). I agree with the author that they provided good guidelines about what enemies should do.
Morale could be a good rules option, and even could be an "optional option": one could use morale, except in certain circumstances (such as a climactic battle).
On the other hand, in modern game design, I fully expect that if the game has morale rules, there will be feats, spells, items, and other effects that interact with them . . . thus creating the "morale check build". That would be a sad, and predictable, consequence.
I disagree. We may never know the frequency of morale rules being simply because this depended on individual groups. If I say my D&D group didn't use morale rules (or even if I knew several groups that didn't use the rules), I can't say that "morale rules weren't used frequently." As far as the rules and rulebooks were concerned, morale was a general rule listed in the DMG, and exceptions to the general morale rules were listed in supporting materials. For instance, certain entries in the "monster books" can be found for monsters that had bonuses or penalties to their morale checks:
MM1 = Monster Manual
L&L = Legends and Lore
FF = Fiend Folio
(Couldn’t find my Monster Manual II)
P. 53 (MM 1) - Hobgoblins get a +1 bonus to their morale
P. 67 (MM 1) – Bandits get a +1 bonus to morale, Berserkers “never check their morale.”
P. 68 (MM 1) – Cavemen receive a -1 penalty to morale, Dervishes “never check their morale.”
P. 76 (MM 1) – Orc leaders get a +1 to morale.
P. 49 (L&L) – Minions of Set never check morale.
P. 106 (L&L) – The name of “Thor” is enough to make giants check morale.
P. 9 (FF) – Achaierai never check morale in groups.
p. 26 (FF) – Dire Corby never check morale.
P. 42 (FF) Giant striders never check morale.
P. 43 (FF) Gibberlings cause hirelings to check morale every round.
P. 67 (FF) – Necrophidus never check morale.
P. 77 (FF) – Sandmen never check morale.
So, while the "monster books" didn't include the rules (as a system) for morale, they did mention "special cases" or exceptions to the rules as they were printed in the DMG.