In many ways 1960's and 70's sitcoms such as I Dream of Jeanie and Bewitched set the stage for the urban fantasy genre. There was one mortal and one magical partner, and the magical partner stove to act “normal” for the sake of love. There were a few situations when the magical partner was called on to act for the greater good, but the main theme of both shows involved the struggle of the supernatural partner to please by trying (and failing) to hide their true talents. Darrin from Bewitched was particularly adamant that his wife spend her day cooking and cleaning, and that she never use her power to benefit either one of them. Even though I was quite young when I watched Bewitched, I thought there was something extremely wrong with Darrin's reasoning. Every so often I still run across this theme in a subplot in an urban fantasy (or more likely in a plot of a paranormal romance) and I can't help but wonder why such a trope is still being used.
Generally speaking, a potential mate's special skills and abilities are one of the reasons a person is attractive. Can you imagine a woman marrying a profession basketball player, and then demanding that he give up sports and fame (and the money that comes with it), and instead work as a “normal” person in a some low-level clerical cubical for minimum wage? Did Melinda Gates demand that Bill give up business and computers (and their fortune), and instead work in construction so that he not stand out for his particular skills? Although one might see an argument for trying not to abuse magical powers or be outed in a society that may not understand those powers, it seems odd that so many urban fantasy and paranormal romance characters should try to insist that a magical person deny who they are in the name of love.
There are a few good reasons for writing characters who wants a supernatural partner to avoid certain aspects of their nature. For example, the recent popularity of vampire lovers leads to very necessary questions about vampire's killing people and the drinking blood (and these are usually dealt with by the vampire giving up killing and making other arrangements to feed). There are also times when the person with power feels such responsibility that they would themselves like to put it aside (as Buffy the Vampire Slayer was sometimes prone to do). Finally, the selfish use of power might be a turn off to those who don't want to play moll to a super villain. These all all legitimate reasons for star-crossing an interspecies paranormal romance. However, even in the age of the kick-ass heroine, magical people are often asked to give up magic simply because a human love interest is uncomfortable with it (though in modern stories the man is just as often asked to give up power for a female). Sometimes even when both partners are supernatural, the less powerful one demands the other start acting as if they don't have the extra advantages.
I wonder if this “Put your lover's light under a bushel” or Darrin Syndrome trope is some sort of literary application of the “ladder theory” of romance. According to the Ladder Theory, everyone ranks every potential partner in terms of the level of attraction, and trades up whenever possible. If this were the case, then in world where supernatural powers can add to the individual's rating, a mortal would do well to (a) snag someone more powerful and attractive than themselves and (b) do everything possible so that neither the higher ranked partner nor that partner's other potential mates realize that the magical partner could do better. Seen from this perspective, it gives our heroes and heroines poor self esteem and control issues.
Let's take a cynical view of the one of original trope sources: Bewitched. If Samantha Stevens hadn't spent all day vacuuming and dusting, then she may have realized that there were many hotter, richer warlocks who could offer much more than Darrin. She may even notice there are quite a few humans who would give a good looking young blond like her a maid and a nanny (and not ask questions such as "Where did the swimming pool come from?"). Maybe this is the point Endora was not so subtly trying to make when she objected to What's-His-Name. In many ways, Samantha's mother did Darrin a favor by throwing up opposition to the marriage. Samantha's desire to rebel may have stopped or overridden her own realization of Darrin's lower ranking. Then again, perhaps Samantha was so insecure that she married low enough to ensure he could never replace her with someone of higher ranking.
Perhaps it's best not to look too deeply into the motivation of fictional characters (especially not those in sitcoms). All the same, part of the fun of being a fan is identifying with or living vicariously through our favorite characters, so fans come to have certain expectations and opinions. Much can be forgiven in a good story, however, modern characters who fall prey to this trope are still perplexing. I will overlook brief and occasional outbreaks of the Darrin Syndrome in my favorite books or shows, but I will be secretly hoping they find a cure for it and soon.