For what I understand, unlike trademarks, patents don't have a provision that you should protect them or enforce them. In fact that's why defensive patent portfolios work, isn't it?
Laches is a form of estoppel, which is geared around the basic idea that if you do something I could sue you over, and you can show that:
1. I did something which gave you a reasonable belief that I wouldn't sue you, and
2. That was the only reason you went ahead and did it, then
3. I don't get to turn around and go after you, because it was really my actions which created the situation.
In a laches defense, you show that the person suing you for patent infringement knew you were doing or were going to do something which infringed, and deliberately waited to come forward with their claims so as to do the most possible damage to you. You argue that, had you known about their claims earlier, you would have done something different, and so it was their inaction which led to the current situation.
This doesn't stop the patent holder from going after other people, so long as they do it in a timely manner; the patent is still valid and can still be enforced. It also doesn't automatically stop future claims against you; it's expected, now that you're aware of their patent, that you'll work out some licensing deal to avoid future infringement. But it can throw out their current case against you, or at least throw out the massive back damages you'd owe from infringement over the period when the patent holder didn't bother doing anything.
I disagree. In Symbol Technologies Inc. v. Lemelson Medical, the courts declared Lemelson's submarine patents unenforceable under the doctrine of prosecution laches.
Apparently cases called Aukerman and Scimed established a precedent of 6 year for invoking a patent claim. "[A] delay of more than six years after the omitted inventor knew or should have known of the issuance of the patent will produce a rebuttable presumption of laches."