While there is no requirement to issue a DMCA takedown notice, there is the possibility of a defendant in a copyright suit collecting attorney's fees and court costs in a successful defense.
A website owner who (1) has properly obtained the DMCA liability shield* and (2) can show that the infringing material was placed on the site at the direction of a user** has a complete defense to a copyright infringement claim. While courts are sometimes hesitant to grant attorney's fees to prevailing copyright defendants, they are available under the statute. I'd think that anyone with a valid DMCA defense (particularly if it is coupled with another defense like fair use) would have a decent chance of convincing a judge to award fees and costs to the defense due to the frivolous nature of such a suit.
Of course that means you still have to pony up to pay for your defense in the first place...
* Not as easy as many people think: be sure to closely follow the instructions in the statute and on the Copyright Office website.
** This takes most bloggers and other website owners out of the DMCA's umbrella if they post the material themselves
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