Continuing from last week, I'd like to dissect and reply a comment a little more thoroughly than a replying comment will allow.
While not disagreeing with your issues, I think most of these problems could have been avoided with a single line in the Monster Manual that stated "DMs should feel free to assume that monsters trained in a skill that have an Intelligence greater than 6 can be given and can invoke rituals keyed off of that skill, as necessary for the plot. Don't forget to include the creature's ritual book as treasure."
I think this is pretty much assumed. After all, the Draconomicon includes rituals presumably to be used by dragons, none of whom are specifically described as being ritual casters. The Dungeon Masters Guide could also have had some guides as to when to give out ritual books as treasure. I think some "recommended rituals" could have been included in a monster's generic entry. (Also, it would be nice if more Dungeon adventures gave rituals to the NPCs. As far as I know, the only one that bothered to do so is Keep on the Shadowfell.)
It's a great one-line solution to a lot of monster powers and the lack of monster depth. But let's look at monsters casting rituals a little more.
Allowing monsters access to rituals is a quick and simple way to give monsters additional options outside of combat. Arcane and intelligent monsters make great ritual casters and adding the results of their rituals to lairs adds some nice description and flavour (and their ritual books eat-up treasure nicely).
Some creatures cannot use rituals. For example, a heavy part of the fluff of beholders in older editions was that their central eye created an anti-magic field, preventing them from being wizards. Beholder mages had to partially blind themselves. Simply adding rituals to replace their lost eye-charm seems contrary to the monster's roots. Compromising the integrity of the world or the story for the rules should be a final option. But there's a simple fix for this. For non-intelligent or non-magical critters it's easy to fluff/change rituals into an innate ability, akin to the bard class feature to cast small rituals.
The type of rituals know, or the specific rituals, can do a lot to add flavour and interesting hooks to some monsters. Do they favour elaborate arcane rituals? Divination spells? Charms? Illusions?
As an example, imagine a few gnomish ritual users and how that influences their use. Gnomes might have lairs decorated by illusions and figments: instead of paintings and decorations there might be elaborate magical glamours. Why paint the walls when a single ritual can do it? It also suggest combats involving permanent illusions, phantasms, and the like. While nothing stops a DM from creating an encounter with illusions and terrain, they'll be creating the rules and how it interacts with the players from scratch.
The obvious ritual problem is the publication problem. New rituals are published all the time, so it's possible for a monster's perfect ritual to be published after the monster, with no reference connecting the two. Likewise, it runs into the reference problem. DMs suddenly have to consult a player's book for monster information. All the information is no longer present, just like spell-like abilities from Third Edition.
The big problem with using rituals is that they are for players. One of 4e's big changes for monsters is that they don't use the same rules as players. Rituals are carefully written and designed to not give a player a long-term advantage or unbalance the game. Monsters should (and do) have abilities that no player should have access to, and this applies to non-combat abilities as well.
Rituals also require a financial sacrifice for a bonus, which has no affect on a monster who can have as much or little gold as the DM decides. Gold is just not an adequate limitation on monster power usage.
Rituals also either have very simple and negligible counter measures, or none at all. Monsters and opponents are meant to easily ignore some rituals while being stopped by others. A good DM solution is to apply skill challenge mechanics to overcoming rituals, but this is simply a house ruling.
Complicated and elaborate use of rituals should also affect the level of the encounter and experience reward, essentially being little more than traps. But, again, this would just be a house rule and it could be argued that interesting terrain is just a requirement of encounter design.
Not to trivialize or intentionally single-out anyone for disagreement; just felt monsters and rituals needed a dab of extra attention. It's a large enough topic to warrant examining and I'd probably give it even more detail if my brain wasn't hindered by a cold and cold medication.