Over at Mighty God King, MGK just posited that Deathstroke the Terminator was a much better character back when "[h]e showed up every once in a while, was incomparably badass, and then disappeared for a bit."
I submit that, with a few rare exceptions, this is true of every supervillain out there.
This kind of overexposure doesn't just diminish the villains, it diminishes their heroic counterparts, as well.
It's a subset of Follow The Leader that I call the The Show Biz Bugs Syndrome: "It'th a great trick, but you can only do it wunth!"
When Frank Miller made the Kingpin a fixture in Daredevil, it gave Matt Murdock a focus and a direction that previous writers had failed to instill.
When John Byrne reinvented Lex Luthor as the Corrupt Corporate Executive, it just made Superman look ineffectual. By the definition of Luthor's new persona, Superman was not allowed to beat him. Ever.
By far, the hero who's suffered the worst of this has been the Batman. In the last decade or two, adversaries who once appeared every few years have become members of the supporting cast, crime bosses in Gotham who get more monthly panel time than Jim Gordon or Alfred.
And, as a result, as Batman's wealth and technology and planning ability has increased to ridiculous levels, he's become pretty much useless. except when he's fighting other heroes. The argument that "Batman should just kill the Joker" didn't have as much impact when the Joker got tossed into Arkham (or jail) and we didn't see him again for a couple of years.
For all the silliness of the "Sci Fi Batman" of the late '50s and early '60s, he was far, far more effective than the Grim And Gritty Vigilante of the post-Miller days. When he put someone away, they stayed away, and often even served their full sentence (as I mentioned in passing in Fine Feathered Felony the other day). Late Golden Age Gotham was often touted as a model city for law enforcement, and civilian characters would, on occasion, mention that they'd moved there because it was so safe, thanks to The Batman.
The writers of the Golden Age and Silver Age knew that there were only so many good stories you could tell with a given antagonist, and used them sparingly. There were also much more willing to whip up a new adversary and, well, "throw it against the wall to see what sticks." There's more reluctance to devise new foes in this day and age (Grant Morrison being a notable exception), and I think that, too, is a detriment to both characters and creators.