If you ever hear a cleric say that, it's probably April 1st. Unfortunately, the way D&D is usually played, both magic-users and clerics pretty much always prepare the same damn spells for an adventuring session. The reason is obvious: if you don't know exactly what you'll be facing, then it stands to reason that you'd select those spells that are most likely to be useful against the sorts of things that you usually end up facing: monsters. So clerics take healing spells, or maybe command (which is still risky because you probably won't speak the right language) and magic-users prepare sleep or magic missile.
In my current Barrowmaze campaign, I'm trying to encourage a little more variety by rolling randomly for the clerics' bonus spells (an idea I believe I got from the Akratic Wizardry blog), but it's a bit too heavy-handed in limiting the players' choices regarding a major aspect of their PCs capabilities.
It seems to me that the problem is just that: the players don't know what they'll be facing. By overcoming that lack of knowledge the spell casters would be in a better position to make more nuanced spell choices, and this can be done in (at least) two ways. The players themselves could attempt initial reconnaissance missions before making a head on assault on the dungeon. I understand that Rub Kuntz's Robilar character would first investigate a given level of Castle Greyhawk and get the lay of the dungeon before taking on whatever was there. So if the party scout comes back with info about a room full of snakes, then the cleric could prepare snake charm for the next day's attempt on that room.
Also, DMs could make info about the dungeon available through NPCs that have braved the dungeon themselves and, for the right price, are willing to share a few of the secrets they've discovered with the party. In fact, tracking these sorts of NPCs down might make for an interesting side-adventure. "There's a rumor in town about an old fighter living off in the hills who years ago lost his right arm and was driven to near madness by something he ran into in the dungeons below the ruined castle. If you can find him he may have something useful to say about what's in there." And such info may very well give the spell casters ideas as to what spells to prepare against these threats, that might prove even more useful than cure light wounds or magic missile.