Maybe, with dragons, we need to borrow a leaf from 1E.
In 1E, not every dragon automatically had the ability to assume human form, to cast spells, or to even talk. Every dragon had a percentage chance of being able to talk, or to cast magic. Ever since Dragonlance, that setting's image of dragons as being all-powerul, uber-monsters has spread to the game in general-all dragons have powerful intellects, the ability to polymorph, inherent magical powers, etc.
If a dragon can't cast spells, and can't even talk, doesn't that dragon immediately become easier to use?
And as far as using spell-like abilities, let's remember what we're dealing with...
-Demons are constantly being depicted as bored with their eternal existence, only taking pleasure in tormenting the creatures of the material plane. Where's the fun in simply blasting them out of existence? Why hit them with a death spell when you can hit them with a symbol of hopelessness or pain instead, leaving them crippled and at a disadvantage, slowly killing them and making them fear you, especially when you're essentially an epitome of evil? Mariliths, for example, might take particular pleasure in grappling enemies with their tails, slowly crushing them to death as they simultaneously channel their cause serious wounds ability at the same time for maximum pain, even as they use their swords to take on other enemies.
Instant death=no fun. Slow, painful, agonizing murder=good times.
-Dragons, especially evil ones, love terrifying people. They might exult in their power, and take pleasure in watching the PCs run in terror, before ripping them apart with fang and claw, before taking their wealth back to the lair? Besides which, any wealth or magic they might have could be destroyed by that massive blast of fire or lightning, or the fireworks of the spells they cast.
Dragons might take a perverse pleasure in letting their legends spread, and for making people who live hundreds of miles away shudder at the mention of their names. It might appeal to their ego to have would-be heroes and treasure hunters come trying to slay them, before ending up on the dragon's trophy wall.
Now, with all this in mind, if the dragons or demons realize they're facing sterner opposition than they expected, then by all means have them use their most powerful abilities. And if they know exactly who they're facing, fun times are over and no more Mr. Nice Dragon or Mrs. Nice Marilith. Experienced monsters will know when to indulge their tastes for sadism and when to play rough.
And, of course, both kinds of monsters can be masters of subtlety. Just don't forget that they, like all kinds of BBEGs, don't necessarily have to operate in secret or subtlety, or be hidden masters behind some grand plot.