4E's method of "maybe the designer remembered that Hobgoblins have this ability, maybe the designer forgot" is lacking in flavor and consistency.
This is not a fair description of 4e. In my experience 4e has characterful and consistant design of monsters, particularly types of monsters, who are distinctive in ways that are unmatched in previous editions.
I compare this to the approach of previous editions, which rarely gave most monsters much of anything to do, and the ones that it did, were weighed down with skills, spellbooks and more which didn't serve any gameplay purpose.
I find that simulationist dogma doesn't make for good or evocative battles, and that's why it makes sense to me that every giant doesn't have the same powers as every other giant.
5e monster design should stick to 4e's approach, especially if they (as some people suggested earlier) want a monster sheet to include everything the DM needs to run them.
I don't need codified quasi-story on the monster sheet in my games. I need a combat sheet.
I can make my own fluff, or read the fluff in a context better suited to it (such as knolwdge checks, or a general lore section), which can be written more evocativly as a result, as well.
I certainly don't want dragons-as-sorcerors, in fact i'm flat out finding room for Devils-As-Spellcasters. I remember one of the more powerful fiends in 3e had a sword, and a flaming whip- but their best move by far, was to, inexplicably make people Implode. Why? Because implode is a high level spell they can cast.
Sure, a spellcaster like a lich should be built around spellcasting, but even here, monster design must be unbound by dogma.
When my PCs fought a solo lich, he had a power for each shool of specialization- (including a seeming evocation spelll wich was actually an illusion!)- but doing that wasn't a process of poring over spell lists and trying to shoe-horn existing spells into the monster design- it was based on creating unique powers which FELT and PLAYED like like a powerful, undead spellcaster.
But, this complicates the game system. It's not so much that the Vampire is casting a Charm, but the game system is a LOT more simple if the Vampire Charm and the Charm spell are identical or nearly identical.
This is an argument for common conditions, not common powers. Likewise, having spells instead of powers isn't going to make things more simple, especially if PC-style spells come with a bunch of additional features like interruption of casting, and how often they can be cast in a day.
It makes no sense in a balanced and hence functional 4e approach to tie monster abilities into spells just because that's how they did it in the old days.
It's not a simple claw, claw, bite, it's a specific ability that some werewolves have and others do not and when running different ones together, I as a DM sometimes miss this stuff.
Anything you can miss in a monster power, can be missed just as much if not more in a spell list that carries a bunch of legacy issues.
Exception based power design is a huge asset to the game, and rejecting it is going to cost WOTC customers.