So the dragon expects to fight epic golems? Well if so, it flies, and they don't. So uh, it completely bypasses the encounter?
And it might want to take the golem out for some reason? What if it's defending its home (or something in its home)? What if it's defending a town it likes? What if the dragon hates undead (and dead things in general) and it sees one of those corpse collosi?
Dragons are (on average) more intelligent than people. They might have a slightly alien worldview from our perspective, but that doesn't mean they don't have motivation.
So, a random golem just walks in? Yeah, sure. Whatever. I think that's qualifying as definite edge case. Now Grease is a nice spell, but not at the levels where everyone is flying.
They might want to use those slots on something else. Conservation of resources, man.
I'm not saying something else can't do what silent image can do and then some, but why use a nuke when a bullet will work instead? The dragon might want something else in a higher level slot.
Dragons have Sorc casting, which means a lot of slots. Don't think they're in much danger of running out, even when doing things like spamming Arcane Strike. Also, Action Economy. If you're going to try and fool someone, you'll want to make sure it works. Silent Image is foiled by more things than the higher level images.
And something tells me Grease will not help with... that...