Actually, swordmaker, patent law doesn't require a defense to maintain a patent. I think you're thinking of trademark law, which is similar in many respects. A holder of a trademark (which never expires, unlike patents, and at least in theory with copyright - though the jury is out whether any copyright will ever expire in this country again), must enforce any known infringement of the mark or they can lose their exclusive use of it. Many patent trolls have made millions by letting a patent just sit there for years, so that people unaware of it will use it, (which to me begs why the patent is issued, since anything that is used that widely is likely fairly obvious), so they can swoop in with the lawsuits en mass and make big bucks through government sponsored extortion.
It does require a defense of the patent if you want to stop someone from using it.
I realize that. . . my point was that it DOES take the pro-active defense of your patent to protect it. If the owner does not sue the infringer, the patent is worthless. Unfortunately, a patent infringement suit is very expensive. The patent office will not protect it. Not pursuing it can effect how a court will enforce it as well. Unequal enforcement can effect how the court will value it.
Even the most thorough patent searches can miss obscure claims in non-related patents. That's where a lot of these patent trolls live. Broadly written Claims in unrelated patents that can be construed to be applied to something unrelated.