It looks like the consensus here is entirely unanimous, but...
Well, as a devil's advocate point of view... if items come from some loot table, they are meant to be part of the reward for having defeated the challenge rather than to ramp up the challenge. In fact, it has traditionally been fairly common not to even roll up the loot until the monster is defeated. That's the way it is done in basically all electronic simulations of the hack and slash part of fantasy RPGs... everything from King's Quest, Ultima and Final Fantasy I to World of WarCraft are unaware of what magical items a creature might have because that isn't randomly generated until either the creature is killed or you try to loot the body. The item doesn't exist until it comes time for it to be the prize for you. I believe Skyrim might be an exception, and I am not sure about previous Elder Scrolls games.
And there has only been a single time I have seen a monster use a magical item that was generated off of a loot table... and it was a time when the DM was sick of one player's munchkin character easily breezing through all challenges, even taking out monsters with ECL above his class level with ease. And the DM thought he found a perfect monster to crush that character by using ambush and webbing and such... and yet, it still wasn't working and so in absolute desperation the DM rolled up the loot for the creature to determine if the creature might have anything at all that could kill this character. And it turned out that one of the items worked and the DM succeeded in killing the character followed by an argument that the DM did cheat by doing this.
So the fact that I have only seen this done as part of a pissing match between a player and a DM about who could "win" without technically breaking any of the written rules of the game by bending them as far as they could go and using every loophole and badly designed element in official splat books... can't say that having the creatures use the potential loot against the players in order to successfully kill the players when the monster's own default stats would not suffice to do so, you aren't going to be left with happy players.
But, from a rational stand-point, a lot of the time if a creature has an item that is magical and would give them an advantage, then they would likely use it so it seems silly to have it turn out that the monster had something potential powerful and deadly yet did not use it when fighting to save its life... But, there are a few times I could see it happening....
If there is a potion, be it an invisibility potion or a healing potion, it is entirely possible that an average or lower intelligence creature in possession of the said potion is actually unaware of the effects of this potion and so would avoid drinking it in case it was a poison or would turn them into a mouse or something.
Or if it is an Ogre or a Giant, as we are to presume that any magical armor or weapon they would possess that is meant to be usable by the PCs later would simply be too small for them. Even if the weapon has a higher bonus in the hands of a medium or small sized creature than their own non-magical weapon, it is likely that they would still prefer their nice big, weighty, meaty weapon-- and in their hands that bigger, heavier weapon might be far more effective anyway. I think if one were going into battle, it would be perfectly reasonable to prefer a nice, hefty hand and a half sword over a letter opener even if it is a letter opener that could cut through things with the same ease as a lightsaber.
Overall, the best advice is to just consider things on a case-by-case basis. If it seems like an item that the creature would likely use a non-magical version of and you are concerned that it is disconcerting that the creature did not use their best weapon in the fight... have them use it.
If it feels like you are making the game more lethal and less rewarding for the players by ramping up the challenge or the creature using the item would mean removing it from the rewards for winning the battle and you can come up with some reason, even slightly convoluted, that the creature might not use the item even if they possess it, by all means have the creature not use the item.