The other school of thought appeals to human cynicism. This other school questions whether a waiter or waitress is really giving you an informed recommendation or simply pulling a "recommendation" out of their hats. Or, even worse, recommending a dish that the restaurant has too much of -- i.e., nobody seems to be ordering the skate in ginger sauce or the poulet normande so the servers are told to recommend these dishes.
I'm caught somewhere in between these two schools. First, I think it depends a lot on the type of restaurant in which you happen to be dining. If the menu is fairly static -- no seasonal shifts or daily specials -- there's less reason to ask about recommendations. Second, I pay close attention to what my server says about the recommended dish. I tend to be suspicious if all they say is: "I recommend the sea bass." I usually want to know why they recommend it. Is it fresh? Is it a specialty of the house? If they can't offer me a little more info, I take their recommendation with a grain of salt (yes, pardon the pun).
Last but not least, I take a cue from the tone of the server's recommendation. If they seem tentative or as if they're reciting a couple of dishes from a list, I tend not to be swayed. But if they sound confident -- like they really want to steer me toward a dish -- I'm more likely to listen. After all, they have to know that the amount of my tip (to some extent) is shaped by whether the food they praised or pushed actually met my expectations.
Deciding how much faith to put in a server's recommendation would be easier if I had a good sense of how many waiters actually eat or try the food in their establishments. I think most servers do eat or try the dishes in top-flight restaurants, but that's just my guess. It's hard to know. If they aren't, there's a problem because waiters can't truly recommend a dish that they haven't tried, can they?